The tension showed with three first-half cautions before Latics boss John Sheridan was despatched from the dugout by West Midlands referee Andy Hall seven minutes before the break.
Ex-Rover Reuben Hazell gave Oldham a big lift with the opening goal on 35 minutes. Andy Liddell provided the cross following a free-kick and Hazell headed in at the far post with keeper Danny Coyne getting a touch but being unable to prevent the ball going in.
It proved enough to earn all three points for the Latics.
Antony Kay had no luck with two early efforts for Tranmere and the home side was almost caught napping on 18 minutes following a long clearance by keeper Mark Crossley.
Coyne managed to deny Craig Davies with an outstretched leg.
Five minutes later Tranmere could have been ahead, but Crossley did well to turn aside a 20-yarder by Bas Savage.
Craig Curran and Chris Shuker missed chances to equalise just before the break after good home approach work.
The visitors came close to doubling their advantage soon after half-time, but Danny Whitaker fired just over from the edge of the box before a deep Liddell cross struck the far post.
Andy Taylor blasted over with a free-kick for Tranmere before manager Ronnie Moore changed his formation as he sent on strikers Chris Greenacre and Ian Moore.
Oldham's Chris Taylor was unlucky to have a goalbound shot deflected away by his own player Whitaker on 69 minutes. Soon after Crossley saved well from Edrissa Sonko at the other end.
Oldham's Kelvin Lomax and the Tranmere duo of Kay and Curran collected the first-half cautions and there was no let-up later with Oldham's Crossley, Sean Gregan and Neal Eardley, along with Tranmere's Ben Chorley, Sonko, Steve Jennings and Moore going in the book.
Tranmere thought they had equalised on 80 minutes when Jennings bundled the ball home from a Savage cross but offside was given.