His elder brother James, who inherited his father's estate of Dunnikier, was MP for Dysart Burghs and Fife between 1741 and 1768 and a Lord Commissioner of the Treasury.
[5] His grandfathers were James Oswald (elder) of Dunnikier, MP for Dysart Burghs (1710–1715),[6][7] and John Durie, who was Provost of Kirkcaldy in 1704.
[16][17] Captain Oswald was based in Reading, Pennsylvania, which commanded a line of forts defending the English colonies in America during the French and Indian War.
[18] He commanded a company under Colonel Conrad Weiser defending the frontier and taking part in the interrogation of French prisoners in late 1757.
The rear of the left flank, facing French irregulars on that side, was covered by two battalions commanded by Brigadier-General George Townshend.
Within days, on 18 September, Jean-Baptiste Nicolas Roch de Ramezay, for the French, and Admiral Sir Charles Saunders (Royal Navy officer), and General Townshend signed the Articles of Capitulation of Quebec and the city was turned over to British control.
[25][26] Next year, Major Oswald took part in General Murray's expedition from Quebec to take Montreal and the rest of French Canada.
[35] In 1774, he was brought back from half pay (i.e. reserve) to command the 2nd (Queen's Royal) Regiment of Foot 4 August 1774, still with the rank of Lieutenant Colonel (Robert Raitt deputising).