An Augustinian priory was founded in Tendring Hundred ca.1119 – a little of its early twelfth century fabric still surviving – in honour of Saint Osyth by Richard de Belmeis, Bishop of London, a man of modest birth and (at first) few landed possessions, but whose ambition, capability, and loyalty enabled him to rise through administrative services to the king. Of a worldly outlook, and initially linked to town-founder Roger de Montgomery, Richard steered diocesan preferments the way of members of his family, a like-named nephew becoming a later bishop of London and being succeeded by yet another kinsman [Christopher Brooke, London 800-1216: The Shaping of a City, London: Secker & Warburgh, 1975, pp.34-46]. Bishop Belmeis transplanted in 1121 to his new foundation Augustinian canons from the London priory of Holy Trinity, (the order favouring urban locations), founded (ca.1107) and favoured by Henry I and his queen. The bishop had endowed St. Osyth's priory with the manor of Chich in which it was located, in north-east Essex, a few miles from the coast and just under twelve miles south-east of Colchester, as well as with the parish church. Another endowment supported a chantry there for the souls of King Henry I and Queen Matilda (patroness of Aldgate's priory), thus helping ensure the episcopal foundation and endowments were endorsed and supported by that king. It is not certain if the bishop, ailing in his last years and focused on strengthening the priory, realized his ambition to retire there, but he was at least buried there (1127).

Colchester's was the principal market of the region; the only other commercial competition for St. Osyth being markets at Elmstead and Great Oakley, neither licensed until 1253 and neither very close to St. Osyth, although Elmstead Market may have been founded partly to tap into commerce travelling between the coast and Colchester, while Manningtree's was even farther away and equally late. There was possibly a market at Wivenhoe, though no licence is known for this and Wivenhoe seems to have been a late developer whose main economic asset was a quay on the Colne. There is no indication that Tendring, after which the hundred was named and where met the hundred court, had a market – at least, not a formal one.

The priory – which, before the end of the twelfth century, had been promoted to an abbey (though still commonly referred to as a priory) – had a rare dedication, to a seventh-century Saxon princess who, legend has it, escaped an unwanted marriage by becoming a nun and later founded a convent at Chich, of which she served as the first abbess, later dying a martyr during a Viking raid on which was also blamed the disappearance of the convent. The legend is unsubstantiated and doubtful – one of the earlier surviving biographies of the saint is attributed to a member of the prominent de Vere family who was then a canon at the priory, ca. 1160; but it sufficed to encourage the historical foundation by the Bishop of London, who added into the dedication saints Peter and Paul (purportedly the original dedication of Osyth's convent). Despite that it was not Chich but Aylesbury, Osyth's reputed burial site, which became a pilgrimage attraction, the abbey named for her was well-endowed and remained wealthy throughout the Middle Ages – despite a period of financial difficulties around the 1430s – while the religious house founded in St. Osyth's honour at Aylesbury did not survive to the Dissolution.

Henry II granted the canons of St. Osyth's a market, though the precise date is unknown; Henry probably made various grants to the priory which were subsequently compiled into a single text to facilitate the confirmations of his successors. We know of them through Edward I's confirmation (1286) of his father's charter of 1268, itself confirming various undated grants (some as far back as the time of Henry II and perhaps earlier) that included approval of the episcopal, royal, and other endowments of the canons of St. Osyth of Chich; the priory was given extensive rights, including exemption from royal tolls, for themselves and their tenants, throughout the king's demesne, and of operation of their market, together with collection of tolls there, without challenge or hindrance. Antiquarians have dated Henry II's grant to 1168/69 [e.g. John Watney, "St. Osyth's Priory," Transactions of the Essex Archaeological Society, vol.5 (1873), p.9; J. Yelloly Watson, The Tendring Hundred in the Olden Time, Colchester, 1877, p.206]; however, the identities of the bishops of London and Ely who witnessed the charter suggest instead a date from the 1170s or '80s. The phrasing of the grant rather suggests that it was endorsing a market already in operation. Another royal concession was that the canons might assart 14 acres of forest in Chich (and other areas in other of their manors) and rent out the cleared land, though we cannot assume this was for purposes of civilian settlement; yet in 1246 the king was querying houses the abbot had erected on assarted land, although this could refer to an initiative at Brentwood (see below).

A grant of Richard I (1189/90) confirmed the canons' right to hold their market without opposition. In 1218 the market-day was altered to Saturday – probably from Sunday – and a Saturday market is heard of again in 1317/18, in the context of a fruitless complaint from Colchester's burgesses about several regional markets they considered detrimental to their own. A fair may also have been granted the priory, but the charter recording this is undated and must be suspect, since it was common enough for religious houses to enhance their status, privileges, or assets by forging documents. Almost nothing further is heard of this fair, although Britnell noted a Colchester dealer in wool and cloth who attended it in 1310, and concluded that it was a wool fair [Growth and Decline in Colchester, 1300-1525, Cambridge University Press, 1986, p.45]; there seems no significant evidence to support this conclusion, although it is likely enough the priory raised sheep on its Essex lands and had wool to sell each year, and that many Tendring land-holders did the same. By 1351 wool-smuggling from locations in the Colne estuary had sufficiently increased in scale that the king appointed a commission to investigate and seize goods being shipped abroad uncustomed; it may be significant that one of the two commissioners was a St. Osyth man. A reference in Colchester's Red Paper Book relates to a complaint by some of its burgesses that the abbot had in 1301 taken pledges from them at the fair for their payment of tolls, despite their chartered exemption; this record reveals the fair was held, unsurprisingly, at the October festival to St. Osyth (a little late in the year for it to have been just a wool fair), and the alleged value of the pledges taken suggests the burgesses were trading in bulk. The abbot defended, unsuccessfully, that the burgesses' exemption was applicable only in royal boroughs and market towns, and not in the abbey's manor.

In contrast to Coggeshall abbey's wool fair, which may have stimulated cloth manufacturing, there are few indications that the harvesting of wool gave rise to any sizable cloth industry at St. Osyth – perhaps partly due to its proximity to the manufacturing centre of Colchester. A Hugh le Webbe of St. Osyth, who transferred land in Chich Rydel in 1324, is the only resident for whom we might posit (and that, very tentatively) a cloth-making occupation. The little evidence that exists of cloth having been sold at St. Osyth concerns only a modest quantity, while a large purchase of cloth by the abbot of St. Osyth in 1460; for the purpose of clothing his household servants, was not made locally but from London draper and alderman Thomas Scott, who in 1463 was suing a succeeding abbot for the unpaid residue of the debt. On the other hand, the terms of reference in the life appointment of an aulnager in 1315 (reiterated 1346 for his successor) specified a list of cloths with which he was to concern himself, and the names of counties and towns associated with their production; St. Osyth appears on this list, suggesting it was known for some cloth specialization, although quite what is not clear from the text. Furthermore, among the few St. Osyth's residents specifically identified in royal records, we encounter a Geoffrey le Taillour (1314), accused of an assault at St. Osyth, along with William Baynard of Chich St. Osyth and William's like-named son and brother Richard; the last may well be the Richard Baynard de St. Osythe apprenticed to a London mercer in 1310, perhaps related to the Henry de St. Osyth, alias atte Swan, who in 1312 was farmer of murage revenues within a London district – his sureties for the farm being two dyers – and who in 1319 paid subsidy in the city's Castle Baynard ward (although note that members of a family named Baignard had landed interests in St. Osyth in 1312). Henry had to obtain a pardon in 1319 for maladministration when murage farmer and keeper of Queenhithe, but in 1333 was nonetheless able to secure a life grant of the office of clerk of the statute merchant at Oxford. Apart from reference in 1414 to an indebted fuller of St. Osyth, there is, however, no indication of cloth-finishing activities and the place attracted only a couple of the Low Countries immigrants – many active in the cloth industry – who were naturalized in 1436.

We may note that the London cases mentioned above are among the rare instances of St. Osyth occurring as a possible surname – something that would have been unlikely to happen until St. Osyth took precedence over Chich as a place-name. Hardly surprising that it is at London the surname occurs, for that great city would be a likely destination for St. Osyth tradesmen looking to improve their prospects. Yet the two above-noted bearers of the surname are practically the only ones found in London records, though there are a very few other indications of interaction between London merchants and St. Osyth men, one of the latter being chapman Thomas Brownyng (1461). If few St. Osyth men are visible at London, nor are they conspicuous at Colchester, which might be expected to serve as another magnet for the ambitious. Between the early fourteenth and mid-fifteenth centuries, thirteen entrants to Colchester citizenship are identified as originating at St. Osyth, one being a butcher and another bearing the surname Chapman (1383/84) – though the fourteenth century saw surnames becoming an increasingly unreliable indicator of occupation, as illustrated by a John le Chapman senior of St. Osyth, mentioned in 1319, who is described as a chaplain, and a John Marchaunt of St. Osyth, mentioned 1345 and also a chaplain (possibly the same individual as Chapman sen., or the implicit John Chapman jun.). The only Colchester entrant of note was John Sumpter (1396/97) who seems to have built wealth and status through a legal practice – his clients including a de Vere dowager countess of Oxford – administrative skills, and marriage into the landed gentry, rather than through trade, and who served Colchester as bailiff and parliamentary representative. Sumpter was implicated, along with the abbots of St. Osyth and Colchester, in an alleged conspiracy (1404) of the countess to overthrow Henry IV; he was cleared by a trial jury, but the abbots had to acquire pardons.

The canons of St. Osyth later acquired market rights on the other side of Essex, when (1227) granted a Wednesday market and short fair in July at their vill of Brentwood, whose site, heavily wooded by the Middle Ages and absent from Domesday, lay on a Roman road connecting London to Colchester. In 1221, when a provisional licence seems to have been obtained for the market, Brentwood was described as a 'new place' belonging to the priory, for it had rented out plots along one side of the through-road (High Street); in 1234 the lord of the neighbouring manor, Thomas de Camvill – already licensee of Fobbing's market (1227) – had royal permission to lay out residential plots along the opposite side of the road, an indication that Brentwood's market was doing well. In 1347 St. Osyth's abbey was granted a licence for a Thursday market at Stowmarket (Suff.), situated on the River Gipping and on a road route between Ipswich and Bury St. Edmunds, and for a fair at the festival of SS. Peter and Paul. Except for the fair, the canons were not founders there, for a market in the royal manor of Thorney, in Stow Hundred, is mentioned in Domesday and the earliest licence for that market was taken out by Richard de Amoundeville (1338). But Stowmarket's church had been one of Henry I's endowments of St. Osyth priory, which is later seen holding a manor there, through a donation by the de Veres; in 1348 the abbot of St. Osyth and the Amoundeville family were in legal contention as to who owned Thorney and the Stowmarket.

The village of Chich – only later known as St. Osyth – stood on a spur of land that overlooked, and sloped southwards to, a tidal tributary of the mouth of the Colne estuary, the St. Osyth Creek, which ran into a larger creek before the latter entered the Colne. The village name may derive from 'creek', or a 'bend' therein, and its raised site would have protected it from seasonal flooding of that creek. Otherwise, the surrounding landscape was largely flat and low-lying, with parts marshy. Settlement was spread out from the priory, in the north-western sector of the site, along a road bound for Colchester. Immediately outside the priory entrance was a triangular green, now tantalizingly known as The Bury, which would have been a site where an original market and the fair could have been held, though such a location for noisy activities must, over time, have become an annoyance to the canons – something similar is suggested at Battle and is explicitly documented at Hinton Charterhouse (Somer.) in 1345, when the Carthusian Priory, having obtained a fair from Henry III, but finding it had become too raucous and boisterous an event, exchanged that licence for another authorizing a fair at Norton St. Philip, a couple of miles further south, and thus away from the ears and eyes of the monks. Markets, we may imagine, would normally have been somewhat less intrusive on the senses than were fairs. It is not evident that there was any civilian settlement (i.e. of traders or artisans) on the opposite side of St. Osyth's green. Settlement instead seems to have grown up immediately south-east of the priory, around a crossroads where the Colchester road, skirting the priory park, was crossed by an east-west road linking the coast near Great Clacton, another manor of the Bishops of London, and the Colne estuary at Point Clear, to the west of the creek. The extent of the priory site restricted this settlement to the opposite side of the Colchester road, though it also extended a short distance along the coast-bound road.

A second focus of settlement was a few hundred yards to the west, beside the creek, and along one side of the road connecting the creek to the Colchester road, up to the point where the priory site blocked further settlement. A wharf was, at some point, built on the bend in the creek; surviving remnants of its fabric have been radiocarbon-dated to between late fifteenth and seventeenth centuries, but this does not preclude there having been an older quayside; alternatively, smaller cargo vessels, such as barges, may have been drawn up on the muddy beach itself. In 1323 St. Osyth is named in an extensive list of ports that includes quite minor places, and in both 1325 and 1326 was among numerous east coast communities ordered to contribute ships towards a royal fleet. By 1360 commercial traffic frequenting St. Osyth's port was sufficient to warrant its inclusion within the jurisdiction of a customs collector based at Colchester. In the sixteenth century there is evidence of moderately large ships involved in coastal and London trade reaching the St. Osyth wharf, while customs accounts of 1481 record one ship (or two bearing the same name) of St. Osyth importing foreign goods at St. Osyth, and in 1393 a St. Osyth ship of the same name was importing wine from Bordeaux, although this cargo was not landed at St. Osyth. Exporting activity is even less well evidenced; in 1375 a St. Osyth man was licensed to ship moderately large quantities of wheat, malt, and flour to London, though Dunwich was used as the port of lading. The port facilities at St. Osyth found a different use in 1473 when the Earl of Oxford attempted, unsuccessfully, to land a small invasion force there.

The parish church, whose oldest fabric dates to the twelfth century, was located at the southern end of the St. Osyth stretch of the Colchester road, a little south of the crossroads; its dedication to SS. Peter and Paul suggests either the priory or the bishop as founder. We might expect that, in what would become one of the largest parishes of Tendring Hundred, the Anglo-Saxon community would have been served by some kind of church, though Domesday gives no indication of such. Yet it would not necessarily have stood on the site of the medieval church; possibly, eleventh-century residents worshipped at the church in the nearby royal vill of Brightlingsea – like St. Osyth a bifocal settlement with a waterside component (and a subsidiary member of the Cinque Ports), but showing no sign of a formal market – or the church of Great Clacton. A document of ca.1050 makes reference to the site of St. Peter's minster, though whether this was an earlier building on the site of the later parish church, or a remnant of the nunnery founded by St. Osyth is unknown.

Between church and crossroads was another triangular space, adjacent to the churchyard and extending between the Colchester road and The Bury. This represents an alternate possibility for the location of the marketplace. It is now known as Church Square and has lost some of its original space and shape due to infilling; a couple of shops are mentioned in a property transfer of 1305, and again changed hands in 1371, but we do not know where in St. Osyth's they were located. A test pit dug in 2004 near the church/marketplace boundary encountered a cobbled surface associated with medieval pottery. We might postulate, therefore, that at some point – perhaps around the time that royal approval was given the market at Chich – the priory transferred the market's location, to distance its noisy activities from their community, and encouraged civilian settlement to refocus around the new marketplace, crossroads, and church – the last conceivably erected around the same time. Whether this was tantamount to foundation of a burghal component is questionable, yet this area was evidently the main focus of the civilian community and, on the through-road, south of the crossroads, a timber-framed building of late fifteenth or early sixteenth century has the form typical of a guildhall and may have served a socio-religious gild dedicated to Holy Trinity, which had ceased to exist by 1551. The date of any such reorganization within Chich is almost impossible to pin down, other than to suggest it occurred some time in the twelfth century. On the other hand, we may note that in 1205 King John granted to St. Paul's and the Bishop of London, then William de Sanctæ Mariæ Ecclesia (1199-1221) – notwithstanding his father's charter grant that the canons should elect their own abbot – the advowson of the abbey of St. Osith, and related regalian rights during vacancies; William played a town-founder's role at Braintree and Chelmsford and it would be tempting to speculate that he might have applied such proclivity at Chich during a vacancy; but this would be to stretch a hypothesis too far beyond the bounds of concrete evidence, despite that Bishop William passed his retirement years at the abbey, which might suggest some involvement with its affairs during his lifetime.

It is likely enough that the St. Osyth market's primary purpose was, at least initially, to serve the needs of the priory community as both consumer and producer of marketable goods; in this context, we may note that among the abbey's holdings by the fourteenth century was a manor in the liberties of Colchester, at Mile End. But the market would also have come to service much of Tendring Hundred, particularly the southern part, in which Chich was situated. Cereals, dairy products, and timber (fuel for London) would have been a regular part of the market's business; in 1251 the abbot was fined by the king for failing to uphold his right, and responsibility, to ensure local bushel measures conformed to the national standard. We may also suspect that fishmongers were active in the marketplace, for oyster beds are known to have been a resource for the local economy, and possibly this was the reason for the quay on St. Osyth Creek; Chich was considered to lie just beyond that stretch of the Colne over which jurisdiction was granted, along with its fishery, by Richard I to Colchester. Oyster shells have been found in many archaeological deposits and suggest them not merely a trade item but a prominent component of the local diet. The value of the fishery to the local economy is indicated by the fact that St. Osyth was one of the coastal communities of north-east Essex to join Colchester in petitioning the king against its long-time opponent, Lionel de Bradenham, when he attempted to control the Colne fishery by blocking off several of its creeks – some of which passed through his own Colne-side manor of Langenhoe – and harassed fishermen operating there, as well as (it was further alleged) abusing his powers as a justice of labourers to support his intimidation efforts and to line his pockets. This complaint prompted a royal enquiry (1362) and resulted in Bradenham's removal from the justiciary but with no other punishment or restraint imposed, and it was left to Colchester alone, now claiming exclusive rights over the Colne fishery, to pursue the matter through the admiralty court, during whose proceedings the abbot of St. Osyth and others were also alleged to have staked off parts of the estuary, thus infringing what the original petition had claimed were public fishing rights. The admiral adjudged that all such enclosures be removed, though Bradenham eventually exculpated himself by purchasing a royal pardon. [Bradenham's career has been recounted in detail by W. R. Powell, "Lionel de Bradenham and his siege of Colchester in 1350," Essex Archaeology and History, vol.22 (1991), 67-75]

Another contributor to the local economy may have been a mill, mentioned in Domesday, on one of the non-episcopal manors. It was also likely situated on the creek, as certainly was a successor, powered initially by the tidal flow, though later by a pond created by damming the creek with a raised gravel causeway which also carried the road across the creek and on to the estuary; this may have been contemporary with the construction of the aforementioned wharf. It is conceivable that some ship construction or repair was undertaken at the creek, although the earliest concrete evidence for this is post-medieval. There is no archaeological trace of industrial activity elsewhere in St. Osyth.

Unfortunately, little documentation relating to the lay community survives from the Middle Ages – even the place-name St. Osyth, which only gradually supplanted that of Chich, is not documented until the hundredal inquests of the reign of Edward I. Consequently, we have only a very hazy picture of the occupational range within the community. Instances of men with surnames suggesting commercial activity (albeit sometimes misleading) have been noted above, and a William le Chapman of St. Osyth is referenced in 1326, while a local fisherman is named in 1436 and a mason, formerly of St. Osyth, three years later; but this kind of information is thin on the ground, and most of it identifies farmers. Nor has much archaeological excavation taken place within St. Osyth, although there have been occasional minor interventions by the Colchester Archaeological Trust [e.g. An Archaeological evaluation at Old School Chase, St Osyth, Essex CAT Report 43 (Sept. 1999], and the results of more extensive investigations by Time Team in 2004 have been assessed and reported [Wessex Archaeology, An Archaeological Evaluation of the Town of St. Osyth and an Assessment of the Results, Report 55753.01, 2005]. Though these investigations found small amounts of pre-medieval material, most of the ceramics found – dominated by Colchester-type ware, some possibly of local production, with little sign of wares imported from far afield until the post-medieval period – suggest that the Late Middle Ages may represent the heyday of the settlement. Documentary evidence tends to give a similar impression, though not a particularly strong one, nor is there any convincing indication of burgage tenements at St. Osyth, but again this may be due to the sparse evidence.

Saxon finds from St. Osyth, though few, serve to confirm human presence between the eighth and tenth centuries, while Domesday Book evidences a fairly sizable population (though no religious community) prior to the Conquest, with the episcopal manor the largest of the three there. A second of the manors, held in 1086 by Count Eustace of Boulogne, was gifted to the priory during Stephen's reign, while the third, part of the Honour of Peverel at Domesday, came to be known as Chich Rydel, after the Gascon family that held it in the thirteenth century and gave or sold parts to the abbey – this looks to have been an area near the creek, for we hear (in the 1230s) of mill, marshland pasture for sheep, and dike with causeway there. All three Domesday manors are portrayed in the Book as having ample pasture for large flocks of sheep, and in 1234 a parcel of 14 acres in Chich Rydel was tenanted by Godwin the shepherd; sheep and goat have been the most numerous of the animal bones found by archaeologists at St. Osyth. The settlement near the creek may have been the focus of economic activity prior to the foundation of the priory. Though population size and wealth had declined after 1066, the foundation of the priory could have helped reverse this. Taxation records of the fourteenth century show St. Osyth among the wealthier Essex communities, though such evidence from the Later Middle Ages points to another decline, due presumably partly to plague and later crises, including flooding when the sea-walls failed in 1381, and partly to the priory's temporary financial difficulties.

The loss of the priory at the Dissolution – its community then including, apart from the abbot, eighteen canons – must have been a blow to St. Osyth's economy, but not necessarily a devastating one, for the other Domesday manors had, perhaps through fragmentation or subinfeudation, introduced well-to-do clients into the parish, so that the abbey's holdings were sometimes distinguished as Chich St. Osyth, differentiating them from other manors such as Earl's Chiche and Chiche Frodewyk. The former was either named after the Count of Boulogne or refers to holdings of the Ferrers earls of Derby; William, Baron Ferrers of Groby, also held Chich Rydel at his death in 1371, possibly consequent to one of the earls of Derby marrying a Peverel heiress. The latter was held of the Ferrers by the Frowyk family, which produced a mayor of London, and later by the Bourchiers as successors to the Ferrer lords of Groby; as early as 1219 a Frowyk is seen holding land in Chich of a Rydel. We also hear of St. Clere's Wick as a farming estate within the parish (though possibly an alias for Chich Rydel), south of the town, held by the de Vere earls of Oxford and, from about the early fourteenth century, of them by the St. Clair family, which acquired some local prominence. Furthermore, the dissolved abbey was, not long after, converted to the private residence of courtier Thomas Darcy, Baron Darcy of Chiche (as of 1551), connected by marriage to the de Veres and, later, to the St. Clairs.

The street-plan of St. Osyth does not appear to have altered much in the Early Modern period, and there are indications of some contraction; the creek-side settlement seems to have ceased after the seventeenth century, though the quay remained in use, and the oyster-fishing continued. Any population growth was accommodated by ribbon development along some of the existing roads, and by infilling of the marketplace. Little industrial activity is seen developing at St. Osyth, and the community gives no sign of being particularly prosperous, nor particularly urban; today it has the status of a village. Yet its combined role of market centre and port may have engendered sufficient commercial vitality to warrant classifying it as temporarily urban. Evidence of prosperity is, again slim, but the bulk of the surviving historical fabric (besides priory and church) dates from the late fifteenth to seventeenth centuries, indicating residents with the means to build, rebuild, or modify, and a few timber-framed houses have yielded specific evidence of such activity.