He was probably born in London, the illegitimate son of Dr William Oldys (1636–1708), chancellor of Lincoln diocese.

His father had 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.

According to the current College of Arms, Oldys was, "a noted antiquary and bibliographer but wholly ignorant of heraldry and known for being 'rarely sober in the afternoon, never after supper', and 'much addicted to low company.

The account of his murder tells of how he was hunted all day due to his support of the monarchy and even though he threw down money to distract the closing pack, he was run through with a sword and died.

The church bell in Adderbury still bears his name and on the wall of the vestry is a plaque written in Latin describing that dreadful day 15 September 1645.