Returning to England, he was for a short time assistant to John Turner (died 1692), an ejected presbyterian, then ministering in Fetter Lane.
He received presbyterian ordination, with three others, at Mansfield on 18 March 1687, his father and his uncle Richard Porter taking part in the ceremony.
Before February 1691 he had become minister of the presbyterian congregation at Oxford, where he renewed an intimacy with Edmund Calamy, begun at Tooting.
He was shy at making friends with undergraduates; Calamy used to get him to meet them at the coffee-house, when "they found he had a great deal more in him than they imagined".
The case went from Coventry to Lichfield, and in November Oldfield went up to London and obtained a stay of ecclesiastical proceedings, transferring the suit to the king's bench.
Here it was argued for several terms; but Oldfield got the matter laid before William III, and the suit was dropped on an intimation from the king that "he was not pleas'd with such prosecutions".
Oldfield left Coventry in 1699 to succeed Thomas Kentish as minister at Globe Alley, Maid Lane, Southwark, a charge previously held by his brother Nathaniel.
He brought his academy with him, and maintained it, first in Southwark, afterwards at Hoxton Square, where he was assisted by William Lorimer (1641–1722) and John Spademan, and (after 1708) by Jean Cappel, who had held the Hebrew chair at Saumur.
Early in his London career Oldfield became intimate with Locke, who was then engaged on his (posthumous) work on the Pauline epistles.
He died on 8 November 1729, and was buried in Bunhill Fields burial ground: funeral sermons were preached by William Harris, and by Hughes.