A goal in each half from Dave Martin and Neil Harris secured Millwall's first League win of the campaign, but The Lions were made to battle every inch of the way in an incident packed, and at times controversial encounter.
Kenny Jackett made three personnel changes and reverted to a 4-4-2 formation after Saturday's goalless draw against Carlisle, with Jack Smith and George Friend occupying the full back positions, Tony Craig coming in at centre back for his first game of the season, and Adam Bolder returning to central midfield.
Alan Dunne, Scott Barron and Neil Harris reverted to the bench.
The early goal his side craved duly arrived after 12 minutes, when Smith's right wing cross was helped on by Steve Morison to Martin at the far post who crashed the ball home via the underside of the bar.
Adam Bolder then went close with an audacious chip that sailed just over and the home side wasted another gilt-edged chance from a three on one situation which saw Morison's cross saved low by keeper Dean Brill with Martin and Gary Alexander waiting on the edge of the six yard box.
Referee Andy D'Urso incensed the home fans and Millwall players on the half hour when he awarded Oldham a penalty for a foul by Tony Craig who appeared to play the ball in a challenge with Andy Holdsworth. But once again David Forde proved to be the hero with a brilliant save to keep out Pawel Abbott's spot-kick.
Forde was again alert seven minutes before the break to deny Keigan Parker.
The Lions passed up two more excellent opportunities early in the second half when substitute Neil Harris headed wide from a Martin cross and Andy Frampton's diving effort was saved by Brill.
Harris was introduced just after half-time for Morison (hamstring) but should have entered the fray on 41 minutes when the former Stevenage striker hobbled off.
However, he was prevented from doing so by the officials who insisted he remove his wedding ring.
Chopper was to have the last laugh in injury time when he nodded the second from close range after fellow sub Jason Price had returned Alexander's cross into his path.
Until that moment, The Latics had remained in with a shout, even though they created few clear cut chances. Some erratic refereeing contributed toward the game becoming fractious, with seven names finally finding their way into Mr D'Urso's book.
The result, however, was all-important and maintains Millwall's solid start to the campaign.

Millwall: Forde, Smith, Craig, Frampton, Friend, Hackett (Price, 83mins), Fuseini, Bolder (Dunne, 87mins), Martin, Morison (Harris, 46mins), Alexander. Subs not used: Sullivan, Barron, Grimes, Laird.
Bookings: Craig (29mins), Fuseini (31mins), Friend (37mins), Harris (77mins)
Scorers: Martin (12mins), Harris (90+2mins)

Oldham Athletic: Brill, Worthington, Holdsworth (Byfield, 74mins), Hazell, Abbott, Furman, Gregan, Parker, Taylor, Marrow, Gilbert. Subs not used: O'Grady, Lomax, Alessandra, Lee, Rowney, Kazimierczak.
Bookings: Taylor (31mins), Worthington (69 mins), Furman (74mins)

Referee: Andy D'Urso
Att: 7,369 (away 163)