His father had also held the office of advocate of the admiralty, but lost it in 1693 because he would not prosecute as traitors and pirates the sailors who had served against England under James II. William Oldys, the younger, lost part of his small patrimony in the South Sea Bubble, and in 1724 went to Yorkshire, spending the greater part of the next six years as the guest of the earl of Malton.
On his return to London he found that his landlord had disposed of the books and papers left in his charge. Among these was an annotated copy of Gerard Langbaine's Dramatick Poets. The book came into the hands of Thomas Coxeter (1689-1747), and subsequently into Theophilus Cibber's possession, and furnished the basis of the Lives of the Poets (1753) published with Cibber's name on the title page, though most of it was written by Robert Shiels.
In 1731 Oldys sold his collections to Edward Harley, second earl of Oxford, who appointed him his literary secretary in 1738. Three years later his patron died, and from that time he worked for the booksellers. His habits were irregular, and in 1751 his debts drove him to the Fleet prison. After two years' imprisonment he was released through the kindness of friends who paid his debts, and in April 1755 he was appointed Norroy king-at-arms by the duke of Norfolk.