Oldfield, who did not join the marines until the close of the American war, served with the 83rd regiment at the siege of Charleston, South Carolinia, in 1780.
He was promoted to a first lieutenancy in the royal marines on 16 April 1778, and, being distinguished by his intelligence and gallantry, was placed on the stall of the quartermaster-general's department.
At the termination of the war Oldfield went to England, and was quartered at Portsmouth, when he purchased a small place in the parish of Westbourne.
The following year he embarked on board the Theseus, 74 guns, and sailed to join the squadron under the orders of the Earl of Saincent of Cadiz.
In the pliant but unsuccessful attempt upon this island Oldfield commanded the force of royal marines which effected a landing from the squadron.
Its failure may be attributed to the loss of the cutter Fox, 10 guns, which was sunk by the enemy's fire, with a considerable part of the force destined for the enterprise.
In a private letter, written after the battle of the Nile, Oldfield said that "it was by no means so severe as the affair at Teneriffe, or the second night of the bombardment of Cadiz."
Until the Theseus was detached to join Nelson (who had shifted his flag to the Vanguard, and gone in pursuit of the French squadron up the Mediterranean), Oldfield remaining with the fleet under the orders of the Earl of St. Vincent.
At the battle of the Nile Oldfield was the senior officer of royal marines in the fleet, and obtained the rank of major for his services, his commission dating 7 October 1798.
Oldfield relates in a private letter how, after the disappointment of not finding the French fleet at Alexandria, the Zealous made the signal at midday on 1 August that it was in the bay of Aboukir.
Early in the spring of 1799 she sailed to join Sir Sidney Smith off the coast of Syria, and Oldfield took part in the defence of St. Jean d'Acre.
His gallant conduct was eulogised in the official despatch of Sir Sidney Smith, and Napoleon, when on passage to St. Helena, spoke of Oldfield's gallantry to the marine officers on board the Northumberland.