Two goals from Byron Webster either side of a Shane Ferguson effort nailed Millwall's place in the League One Play-Offs.
The Lions went into action knowing that they required just one point from their remaining two games against Oldham and Gillingham to clinch a top six spot and goals from defender Webster (4 and 54mins) coupled with Ferguson's 39th-minute goal secured a sixth successive home win.
Boss Neil Harris made one change to his starting XI, opting for the experience of fit-again left winger Ferguson in place of Fred Onyedinma, who dropped to the bench.
Millwall got off to a perfect start and settled any possible nerves with the opening goal after just four minutes. Steve Morison used all his experience to win a corner on the right and when former Latic Chris Taylor whipped in the resulting delivery, there was Webster to send a textbook header into the back of the net.
John Sheridan's Latics didn't panic, however, and responded to the shock of that early goal by showing just why they had strung together an impressive run five wins, two draws and one defeat in their previous eight games.
The Latics, operating with a three-man defence, enjoyed a lot of possession and it took the safe handling of Lions newly crowned Player of the Year Jordan Archer to thwart Cameron Dummigan and Tareiq Holmes-Dennis in quick succession.
Lee Croft could have produced an Oldham equaliser on 18 minutes when he used his robust physique to force his way forward and set up Jonathan Forte, but the latter's shot was blocked for a corner.
An audacious lob from Morison was inches away from putting The Lions 2-0 ahead just moments later as play quickly swung up the other end. Taylor battled for possession on the edge of his own penalty area, threaded a sublime pass to Jimmy Abdou who in turn played in Morison. Noticing that keeper Joel Coleman had strayed forward, Morison despatched a lob from just inside the halfway line, the ball hung in the air for an eternity before dropping onto the roof of the net.
A promising Millwall move involving Mahlon Romeo, Abdou and Morison came to an abrupt halt when Romeo was fouled 18 yards out. Up stepped left-back Joe Martin to take the resulting free-kick, and his sweet effort clipped the angle of the crossbar and upright as the woodwork denied the home side a second goal.
At the other end, a superbly timed Mark Beevers tackle averted the danger after the lively Holmes-Dennis had sprinted into the penalty area as the visitors searched for a way through.
However it was The Lions who eventually did double their tally six minutes before half-time. Morison was again involved with a low cross from the right, Aiden O’Brien’s back heel neatly found Ferguson, who finished with aplomb at the back post.
Taylor came close to adding a third two minutes later against his former club, sweetly connecting with Morison's inch perfect cross from the right, but directing a first-time shot straight into the hands of keeper Coleman.
The visitors came flying out of the traps after the break searching to get a foothold back in the game and twice in the opening three minutes so nearly did. It took the excellent reactions of keeper Archer to block Forte with his feet before the Oldham frontman crowned a miserable afternoon in front of goal thrashing over from close range with the goal at his mercy.
An astonishing run by Romeo saw the rampaging right-back pick his way past seven Latics players, but the angle for his shot at the end was just too tight and he couldn't get enough purchase on his shot, enabling Coleman to collect comfortably at the keeper's near post.
Millwall effectively put the game to bed with 54 minutes on the clock courtesy of Webster's second of the game, this time reacting quickest to Morison's header across goal.
Harris' men really had the bit between their teeth now and produced some slick attacking football, much to the delight of The Den faithful who showed their approval with a rousing rendition of 'Super Neil Harris'.
A finely threaded pass through from Taylor picked out Morison on the hour mark and the skipper broke through on goal, wriggled his way into the penalty area but was crowded out as he prepared to shoot and his effort went straight into the hands of a grateful Coleman.
Taylor's low skidder three minutes later flashed a foot wide of the upright as The Lions continued to look dangerous going forward.
Oldham sub Dominic Poleon conjured up what must be up there as one of the misses of the season, lashing the ball wildly over the bar from two yards after Dummingan's deep delivery had caused a momentary lapse of concentration in the home penalty area.
O'Brien had a great opportunity to add a fourth for The Lions with seven minutes remaining when the outstanding Morison telepathically flicked the ball into his strike partner's path, but the latter’s low shot was straight into the hands of Coleman, who gathered at the second attempt.
Although a fourth goal was not forthcoming, the controlled manner in which Millwall dispatched Oldham was very much a statement of intent as The Lions head into the final League One game of the season at Gillingham next Sunday aiming to keep the momentum going right into the Play-Offs.
Millwall: Archer, Romeo, Martin, Beevers, Webster, Abdou, Thompson, Ferguson (Onyedinma, 68mins), O'Brien, Morison, Taylor (Philpot, 86mins). Subs not used: Forde, Edwards, Craig, Upson, Twardek.