Oliver Groos came from an old Norfolk family which had settled at Sloley in the early 13th century, and he was named after his paternal grandfather, who in 1336 had obtained a royal charter to hold an annual fair there as well as weekly markets at Worstead. When the grandfather’s widow died in 1361, she left properties in London to which Oliver’s father, Sir John Groos, laid claim, but there is no evidence that Oliver himself ever gained control of them. His inheritance of the family manors at Sloley and Irstead was ensured by an entail made by his father in 1372 and the later death without issue of his elder brother John. Subsequently, he also held a manor in Crostwick and lands at Scottow and Lammas, all of which were in the same area of Norfolk, some ten miles to the north-east of Norwich; and before his death he acquired other properties in the south of the county, at Larling and Illington. Through his marriage to Joan White, he eventually came into possession of the lands she had inherited from her mother in Swannington, Alderford and Attlebridge, to the north-west of Norwich.
It seems likely that Groos was trained to be a lawyer, for such an education would help account for his frequent employment as a trustee and executor. But some of those for whom he acted in this way were probably his kinsfolk. His mother may have been one of the Cleres of Ormesby, with whom he himself long maintained close links: he received bequests under the will of William Clere, made in 1384, and from before 1395 until 1429, or later, he acted as a feoffee of the Clere estates (even though his trusteeship involved him a suit in Chancery brought by William Clere’s daughter-in-law).
Like his father-in-law, Groos was retained by John of Gaunt, on whose campaigns he served in Spain in 1386 and in Aquitaine in 1395. The duke granted him an annuity of £10 charged on the manor of Aylsham (Norfolk), which fee was doubled by Lancaster’s son, Henry IV, in September 1400 as a reward for ‘good service’, presumably on the King’s expedition to Scotland in which he had recently taken part. Early in 1403 instructions were sent to the duchy officials in Norfolk that Groos was to receive this income even though he had failed in the obligation to accompany the King to Wales. Nevertheless, by December 1406 arrears amounted to £80, and Groos considered it expedient to forgo half of this sum in order to secure immediate payment of the remainder. As some small compensation he was granted six oaks to assist in the construction of ‘une petite maison’. At the Norfolk elections held earlier that year Groos had acted as mainpernor for John Reymes, the constable of Norwich castle. During his first term as sheriff of Norfolk and Suffolk, he conducted the elections in both shires to the Parliament of 1410. The citizens of Norwich petitioned the same Parliament for a grant of the local subsidy of alnage on worsted cloth, and they gave Groos two marks for his support in promoting their cause.
Henry V had confirmed Groos’s annuities, and in the course of the reign some use was made of his services in local administration. But it was during the King’s absence overseas that Groos was elected to his only known Parliament, in 1419. Ten days after its dissolution he was re-appointed as sheriff of Norfolk and Suffolk and, accordingly, became responsible for holding the elections to the Parliament of 1420. Under Henry VI Groos served once as escheator, and then (in 1426) for a third term as sheriff. In 1430 he made a personal loan of £10 to the Crown. It is not surprising to find his name on the list of those gentry of Norfolk required in 1434 to take the generally administered oath against maintenance.
Evidently a well-respected member of the local community, Groos devoted much of his time to the interests of other East Anglian landowners. William Rees asked him to be a trustee of the manors in Cambridgeshire which he held for term of his wife’s life, and (in 1410) to act as his executor. He was named among the feoffees entrusted by Sir Ralph Shelton (d.1414) with the fulfilment of his will, and he continued to do business on behalf of Shelton’s grandson, John, and the latter’s widow for the next 20 years. More important, Walter, 5th Lord Fitzwalter (d.1432), engaged him for conveyances of estates in Essex and Lincolnshire, and he acted in a similar way for Joan, Lady Beauchamp of Abergavenny, who some time in the 1430s gave him an annuity of £2, perhaps for legal counsel. In many transactions dating from the earliest period of his career, Groos had been associated with Sir Simon Felbrigg KG (d.1443), for whom he appeared as a feoffee of his estates and as an executor of his will. He was also party to the conveyance of a manor in Suffolk to Sir William Phelip (Lord Bardolf).
Groos drew up his will on 1 July 1439, requesting burial in the chapel of St. James in Sloley church and naming among his executors his son-in-law, William Yelverton, j.KB. In June 1440 he made a formal disposition of his lands, authorizing that they should be divided between his surviving sons, John and Rowland, but then apparently lived on eight years longer, dying shortly before 15 July 1448.