St John, Oliver (1559–1630), Viscount Grandison and Baron Tregoz, lord deputy of Ireland, was second son of Nicholas St John of Liddiard-Tregoze, Wiltshire, and his wife Elizabeth, daughter of Sir Richard Blount of Mapledurham, Oxfordshire. He was educated at Trinity College, Oxford, where he matriculated in December 1577 and graduated in June 1578. He entered Lincoln's Inn in 1580, but was forced to abandon his legal training after killing a man in a duel. He fled from England in 1584, served in France and Flanders, rose to the rank of captain, and commanded the mounted troops of the earl of Essex (qv) at the siege of Rouen in the autumn of 1591. On his return to England he was elected to parliament as a member for Cirencester in 1593. In the following years he returned to the continental wars.
St John came to Ireland in February 1600 with the new lord deputy, Lord Mountjoy (qv), in charge of 800 reinforcements. He was knighted by Mountjoy on 28 February 1600 and joined Sir Oliver Lambart (qv) in the north, where he was praised for his action in the field. In late autumn 1601, after Mountjoy had unsuccessfully recommended his appointment as muster master general, he joined the government forces at Kinsale in command of 200 men. He played a prominent part in the siege and was wounded while repelling a night attack on 2 December 1601. Ten days later he went to England with official dispatches. In October 1602 he was posted to Connacht with a company of foot and twenty-five horse to serve under Lambart, who had been appointed president. After the war St John returned to England where he was elected to parliament for Portsmouth in 1604. In December 1605, probably through the influence of Mountjoy, he was appointed master of the ordnance in Ireland in preference to Lambart, who had been nominated by the new lord deputy, Sir Arthur Chichester (qv).
Notwithstanding this uneasy introduction to the Irish administration, St John quickly became one of Chichester's most trusted colleagues and was among the four councillors whom Chichester contemplated taking into his confidence in 1607 when he felt unable to disclose his suspicions of the earl of Tyrone (qv) to the Irish council. In January 1608 St John was appointed to the commission sent to survey the proposed plantation in Ulster, and in the autumn joined a committee in London, composed of leading officials in the Dublin government, which was charged with preparing detailed plans for the plantation. Sometime before May 1611 he was appointed vice-president of Connacht, which in effect made him acting president in the absence of the earl of Clanricard (qv). In May 1611 he wrote a severely critical report on Chichester's acquiescence in the profits made in Connacht by individuals, many of them government officials, who identified the occupiers of ‘concealed lands’ to which the crown had ancient title. In the following October he wrote to the earl of Salisbury to demand a re-examination of all the indents and grants made in the province since the composition arrangements made by Sir John Perrot (qv) in 1585. In the same month a duel was only narrowly averted when St John and Lambart quarrelled about land titles. These incidents led Chichester to regard St John, who was himself an undertaker for 2,500 acres in Armagh, as an enemy of plantations.
In the 1613 parliamentary elections St John was returned unopposed for one of the county seats in Roscommon. It was alleged that the second seat was won by a catholic, Charles O'Connor Don (qv), and that St John forced a new election in which soldiers prevented O'Connor's supporters from voting so that his running mate, Sir John King (qv), was elected. This allegation was subsequently dismissed by a commission of inquiry headed by the lord deputy. When the parliament convened on 18 May St John played a leading role in the disputed election for the speakership in which the official nominee, Sir John Davies (qv), was opposed by an opposition candidate, Sir John Everard (qv).
It was St John, claiming familiarity with the correct procedure from his experience in the English commons, who led the protestant members into the lobby to count the votes, whereupon the opposition took advantage of their absence to place Everard in the speaker's chair. The ensuing fracas was followed by a boycott by opposition members, a protest to the king against the electoral irregularities that had produced a protestant majority, the prorogation of parliament, and the dispatch of rival delegations to London to press the competing cases. Despite their differences on the land issue, Chichester chose St John to lead the group of three who went to London in the summer of 1613 to defend the record of his administration.
St John, having remained in England for more than a year, during which he sat as a member of the ‘addled’ English parliament, resigned from his post as master of the ordnance in December 1614, probably to prepare the way for a bid to displace Clanricard as president of Connacht. It appears that his earlier criticism of Chichester's handling of matters relating to land had brought him to the attention of the reforming opposition faction at court, led by Sir Ralph Winwood, and greater possibilities emerged in 1615 when their protégé George Villiers replaced the earl of Somerset as royal favourite. With Winwood's support, St John raised his sights to the deputyship and attached himself to Villiers, to whom he was already connected by the marriage of his niece to Villiers's half-brother Edward. In October 1615 he was given supervision of the custody of Somerset, and in July 1616 he was appointed lord deputy of Ireland.
Although it was public knowledge that he had been chosen by Villiers, St John was seen as a reformer and was expected to increase the Irish revenue and put an end to the land-trafficking activities of Dublin officials. His commitment to reform, however, was not proof against the demands of his increasingly powerful patron, who was intent on securing control over Irish resources. Thus St John set about establishing clients of Villiers (created earl of Buckingham on 5 January 1617) on the Irish council and in the upper echelons of the administration, as well as facilitating grants of wardships, escheatments and export licences. The revived interest in plantation, which resulted in significant transfers of landownership in Wexford and the midlands, was thoroughly exploited by Buckingham's clientage under the management of St John and the surveyor general, Sir William Parsons (qv).
St John's approach to plantation was both forceful and determined. In Leitrim, where the crown's claim to O'Rourke lands was dubious (and where he acquired his own undertaking), he dispossessed Brian O'Rourke, a minor, by means of a hand-picked jury, and in Wexford objectors were imprisoned in order ‘to terrify others’ (CSPI, 1615–25, 306). It fell to St John to implement the more rigorous policy of religious conformity prompted by the experience of parliamentary opposition. Recusancy fines were imposed on an unprecedented scale, municipal officers were required to take the oath of supremacy, and royal wards were to do likewise on reaching their majority.
In 1621, criticisms in the English parliament of the Villiers connection were abruptly extended to include alleged malpractices in the administration of Ireland. On 26 April Sir John Jephson, an Irish councillor and Munster planter, accused St John of misrule, portrayed Ireland as seething with discontent as a consequence of manifold petty oppressions, and proposed a committee of inquiry. Though this initiative was thwarted, the king was not unresponsive and by midsummer it was public knowledge that a major investigation of the Irish administration and plantations was likely. Buckingham set about arranging St John's recall and persuaded the king to issue a royal warrant on 10 August 1621 which exonerated the lord deputy from responsibility for the alleged defects of Irish government and made it clear that he was not to be the particular target of any future enquiry. To secure his continued influence on Irish affairs, he also procured the appointment of St John's stepson and secretary, Henry Holcroft (qv), as the king's secretary for Irish business. St John was recalled in February 1622, a commission to review the conduct of Irish government began work in April, and St John left Ireland in May.
He had been created Viscount Grandison of Limerick on 3 January 1621. His continued enjoyment of royal favour was signalled by his appointment as an English privy councilor on 28 June 1622. The scathing report of the commission of inquiry left his reputation intact, and he both served as Buckingham's adviser on Irish affairs and, together with two other veteran Irish officials, George Carew (qv) and Arthur Chichester (qv), exerted extensive influence over Irish policy. On 16 August 1625 he was made lord high treasurer of Ireland, as part of a concerted bid by Buckingham to gain control of Irish exchequer patronage; on 26 May 1626 he was raised to the English peerage as Baron Tregoz of Highworth in Wiltshire; and in January 1627 he became one of a quorum of four which largely displaced the privy council committee for Irish affairs. The murder of Buckingham in August 1628 transformed his status. After a period of semi-retirement, he returned to Ireland in May 1630 to assume office as lord high treasurer and to put his Irish estates in order. He died 30 December 1630 at Battersea, England, where he was buried.
He married (c.1592) Joan, daughter and heiress of John Roydon of Battersea, and widow of Sir William Holcroft. They had no children and the barony of Tregoz became extinct. His English property passed to the family of his brother, Sir John St John, and his Irish title to his grandnephew, William Villiers, by special remainder to the heirs of his niece.