While the majority of their first-team colleagues concluded a tour of the Far East on Monday, England trio Joe Hart, Joleon Lescott and James Milner enjoyed their first close-season outings alongside France pair Gael Clichy and Samir Nasri.
Holland's Nigel de Jong took the armband and made his second appearance in four days having started Saturday's 2-1 reserve win at Stockport, while the rest of City's line-up was comprised of academy products and Etihad Stadium outcast Roque Santa Cruz - scorer of his side's equaliser after Robbie Simpson gave the Latics an early advantage.
Having spent the majority of the contest firmly on the back foot, Paul Dickov's npower League One outfit snatched victory in stoppage time as summer recruit Jonathan Grounds rifled home from Connor Hughes' left-wing corner.
City fell behind in the fourth minute when Clichy passed loosely for Oldham to break and Simpson clinically slotted his fourth goal of pre-season with a shot across Hart.
Mario Jelavic, a Croatia Under-19 international on trial with Roberto Mancini's squad, should have equalised but stabbed wide from close range, while Latics goalkeeper Alex Cisak was rooted to the spot when Milner flashed a 25-yard drive past the upright midway through the half.
Jelavic made a better fist of things in the 36th minute, clipping Nasri's 36th-minute pass against Cisak's near post.
The Australian keeper thwarted Milner twice either side of the interval, but could do nothing in the 53rd minute as Santa Cruz rounded him to score after a loose ball fell kindly.
Around the hour City's five returnees made way for a collection of youngsters including Slovakian playmaker Albert Rusnak, who drew two more excellent stops from Cisak before Grounds' late intervention.