With Sir Henry Billingsley and Edward Fenton, in 1588 Osborne was charged by Burghley and Walsingham with the control and audit of the financial accounts of the expedition of Sir Francis Drake and Sir John Norreys against the Spanish in 1589. Osborne continued his work for the Exchequer until his death: he wrote to Lord Burghley in December 1591 urging the continued export of coal. A final decree in Chancery in the case begun in 1588 by Cornelius Avenant against Peter Osborne and Sir Rowland Hayward, as governors of the Company of Mineral and Battery Works, concerning shares in the wireworks at Tintern and other ironworks in Monmouthshire, Derbyshire, and at Isleworth, Middlesex, was not delivered until June 1593. In 1588 also, Osborne, with Ralph Rokeby and Thomas Fanshawe, formed a Commission to approve the construction of four water-wheels beneath a mill-house bridging three sterlings or jettees above London Bridge at its south end, for improving the supply of corn meal in the City: the project was implemented.