This defeat saw the Gills drop to thirteenth in the Sky Bet League One table, but they sit just three points off the play-off places after 12 games.
McDonald, Josh Wright, Jake Hessenthaler
and Scott Wagstaff came into the starting XI after not featuring in the match against AFC Wimbledon last Saturday.
Frank Nouble and Chris Herd also made their first league starts for the Gills after signing for the club on Monday, and the pair combined for the first attempt of the game on seven minutes. Wright played the ball right to Herd, who crossed for Nouble, but the frontman’s header flew over the bar.
Captain Wright was next to have a shot on goal for the Gills. After picking up the ball 30 yards from goal, Wright unleashed a right-footed shot that took a deflection off a Latics player and spun out of play.
Justin Edinburgh’s side took the lead from the resulting corner, with 10 minutes played. Bradley Dack
took the set-piece, and McDonald was left unmarked at back-post and made no mistake with the header, burying it into an empty net with goalkeeper Connor Ripley in no-man’s land.
Nouble was proving a real handful for the Oldham defence, holding up play and driving forward when he was on the ball. With 25 minutes played, he laid the ball to Paul Konchesky, whose inch-perfect cross was met by goal scorer McDonald, but the striker’s header was saved by Ripley in the visitors' goal.
Dack was next to have a shot for the Gills in what was a dominant opening half-hour from the home side. Gillingham’s number 23 struck low from 25 yards, but his shot was gathered comfortably by the Oldham stopper.
Oldham were restricted to long-balls over the top, but Max Ehmer and Deji Oshilaja dealt with any threat expertly, covering their lines and clearing on every occasion.
The Latics’ best opportunity came from a corner with just 10 first-half minutes to play. The ball was played to the edge of the penalty box, eluding both sets of players, where Cameron Burgess latched onto it and struck a powerful shot that Ehmer was able to get his head on and clear for another corner.
Steve Robinson’s side should have been level on 51 minutes. After a goalmouth scramble, the ball ended up at Lee Erwin’s feet at Stuart Nelson
’s back-post. The striker had his back to goal when the ball came to him, and had the simplest of tasks to turn and shoot into an open goal, however his shot somehow went wide, crashing off the advertising boards to the delight of the home fans.
Five minutes later the Gills found themselves in a similar situation. Jamie Reckford looked as though he had a tap-in to level the scores, but his air-shot bobbled wide and out for a goal-kick.
The Gills found it hard to pick up where they left off in the first half, and couldn’t find the same tempo which enabled them to control the game. Oldham looked the most likely to grab a goal, and did just that on 65 minutes.
Substitute Ladapo barged Herd on the edge of the box, but the referee played on, and the forward slotted past Nelson to equalise, just four minutes after replacing Billy McKay.
After a slow start to the second half, the game began to liven up with both teams going for a winner. The Rainham End erupted on 68 minutes after what looked like a hand-ball on the line following Hessenthaler's effort. Referee Kevin Johnson waved play-on to the disbelief of the home supporters; Oldham managed to scramble the ball clear.
Suddenly it was end-to-end stuff at the MEMS Priestfield Stadium, and Ladapo had the ball in the Gills box. The substitute managed to turn Oshilaja, who was back-tracking, but Nelson got down low to block the shot from crawling into the goal.
Latics skipper Paul Clarke should have done better with 77 minutes played, rising highest to head Lee Croft’s corner over the bar.
Substitute Emmanuel Osadebe took the game into his own hands, going on a mazy run down the right-wing, taking on four players in the process before his cross was blocked and cleared to safety.
Oldham had a great opportunity to grab a later winner on 85 minutes. Clarke played a through-ball to Flynn, who cut the ball back across goal, but no-one could get a toe on the ball to stab it into the net. Had Ladapo been an inch closer to goal, Oldham would have taken the lead.
Osadebe again went on a great run for the Gills, this time down the left, with just three minutes of normal time remaining and nearly scored himself, but for a great save from Ripley at his near post.
However, on 89 minutes, Oldham scored the winner. Another goal-mouth scramble led to Flynn picking up the ball inside the six-yard box and poking the ball into the net.
The Gills travel to Bristol Rovers next Saturday in Sky Bet League One, before back-to-back home fixtures against Walsall and Charlton Athletic. Click here
for more information.
Gillingham: Nelson, Herd (Jackson, 69), Ehmer, Oshilaja, Wagstaff (Byrne, 81), Hessenthaler, McDonald, Konchesky, Dack (Osadebe, 66), Wright, Nouble
Unused subs: Hadler, Pask, Oldaker, List
Oldham: Ripley, Reckford, Wilson, Burgess, Flynn, McKay (Ladapo, 61), Winchester (Croft, 59), Erwin (Banks, 90+3), Fane, Clarke, Green
Unused subs: Kettings, Law, Klok, Osei
Referee: Kevin Johnson
Attendance: 6,425 (127 away)