Lambart (Lambert), Sir Oliver (d. 1618), 1st Baron Lambart of Cavan , soldier and Irish administrator, was the son of Walter Lambart, of Preston, Yorkshire, England, and Rose, daughter of Sir Oliver Wallop and niece of Sir Henry Wallop (qv), vice-treasurer of Ireland. This connection probably led to his joining the army in Ireland c.1580. In 1584 he took part in the expedition of Sir John Perrot (qv) against the Scots in Clandeboye, where he was badly wounded and captured by the sons of Shane O'Neill (qv). Anxious to come to terms with the government, they sent him to Dublin with a message to the lord deputy. On his recovery he returned to England, where he joined an expeditionary force to the Netherlands commanded by Sir John Norris (qv) in August 1585. He served with distinction until June 1592, when he was seriously wounded at the siege of Steenwyck and forced to withdraw from the war. In 1596 he took part in the expedition to Cadiz and was knighted by the earl of Essex (qv).
Having returned to the Netherlands as captain of a company of foot in the following year, he was among the veterans who were drafted to strengthen the force that accompanied Essex when he came to Ireland as lord lieutenant in April 1599. When Essex left Ireland in September, he appointed Lambart chief commander in Leinster. He was actively engaged against the rebels in Leix and Offaly in 1600 and was appointed sergeant-major-general of the army before joining Lord Mountjoy (qv) in the north in 1601. On 19 July 1601, on Mountjoy's recommendation, he was appointed governor of Connacht. He took part in the siege of Kinsale (October to December 1601), and later successfully oversaw the pacification of Connacht. Allegations of oppression and of stretching out the war so that he could avail himself of the spoils went unheeded, and on 9 September 1603 he was given a seat on the Irish council and a grant of £100 a year in crown land.
Lambart was a trusted councillor of Sir Arthur Chichester (qv), who became lord deputy in 1605. An anonymous warning of treasonable conspiracy in 1607 identified the two men as the prime targets: ‘these two lights thus put out, they neither fear nor value any in the kingdom’ (McCavitt, Chichester, 130–31). Lambart acted for some years ‘without portfolio’, since Chichester's request that he be appointed master of the ordnance had been denied, and he seems to have been principally called on where confidentiality was required. He was Chichester's confidant in the delicate matter of the 1607 plot, which involved both the Gaelic Ulster lords and members of prominent families in the Pale; he was chosen to carry official notification of the ‘flight of the earls’ to James I; and his attempt to intercept a letter from the secretary of state, Fenton (qv), to London, opposing the lord deputy's plans for plantation in Ulster, was no doubt carried out at Chichester's request. In May 1608 he was dispatched with Sir Richard Wingfield (qv) to quell the rebellion of Sir Cahir O'Doherty (qv) in Inishowen, where he received a slight bullet wound in the right shoulder. In 1610 he was finally appointed master of the ordnance.
During these years, Lambart steadily accumulated grants of land in Wexford, Ulster, and Connacht, where he was associated with Sir Richard Boyle (qv), later 1st earl of Cork, in the discovery of concealed crown lands. In 1609 the commission for concealments endorsed a complaint that he had acquired land belonging to Hugh McTirlaugh Roe O'Connor by underhand means. He played a part in arranging the details of the plantation in Ulster and received a grant of 2,000 escheated acres in Clonmahon, Cavan (where he already held some property), on which he erected a stone mansion and other buildings. A dispute about land seems to have been at the core of a violent quarrel, which came close to being settled by a duel, between Lambart and a fellow councillor, Oliver St John (qv), in October 1611, though the fact that St John had been his successful rival for the post of master of the ordnance in 1605 may have contributed.
Lambart was returned to parliament in 1613 as a knight of the shire for Cavan. His election was unsuccessfully contested by the gentlemen and freeholders on the grounds that it had been conducted illegally. In the election of members for Cavan town, where the majority of burgesses were catholics, Lambart was alleged to have imprisoned one of the catholic candidates, placed an armed guard to prevent catholics from entering the courthouse to cast their votes, and beaten a recalcitrant voter with a truncheon. The government candidates, one of whom was Chichester's secretary, were declared elected, but the result was later overturned by a commission of inquiry. In November 1614 Lambart was given command of an expeditionary force to recover the castle of Dunivaig in Islay from the Macdonalds. Bad weather conditions made the attack difficult, but the castle finally surrendered 2 February 1615.
Lambart was raised to the peerage as Lord Lambart, baron of Cavan, 17 February 1618. He died 23 May 1618 in London and was buried in Westminster abbey (for an outlay of £330). He was survived by his wife Hester (m. a. Jan. 1598), daughter of Sir William Fleetwood of Cardington Manor, Bedfordshire. She died 12 March 1639 and was buried in St Patrick's, Dublin. They had two sons, Charles (qv), who succeeded to the title, and Cary; and three daughters, Jane, Rose, and Lettice.