Bas Savage struck for the second successive away game as The Lions secured a precious point at Boundary Park.
Striker Savage put Kenny Jackett's side within touching distance of a vital win when he slotted home a left-foot shot 12 minutes from time.
But then up popped former Den man Leon Constantine just three minutes later to head home a Chris Taylor cross for the leveller.
On a bittery cold afternoon a swirling wind combined with bumpy pitch made it difficult for either team to play fluent football on the deck.
Lions boss Jackett was yet again forced to make changesm to his side, with Andy Frampton ruled out after twisting his knee in training on Thursday and Marc Laird starting a three-match ban for his red card against Leyton Orient.
In came Zoom Bakayogo to fill the vacant left-back berth, whilst loan signing Jem Karacan came into midfield with Ahmet Brkovic a straight swap for Dave Martin, who dropped to the bench.
The Lions bossed proceedings during the opening half but couldn't find that final precise delivery or composed finish to turn their possession into goals.
Oldham were forced into an early reshuffle when John Thompson went off injured and was replaced by Jason Jarrett - a move that saw former Lion Dave Livermore switch to a central defensive role.
Jay Simpson twice got into good positions after 11 and then 20 minutes, but on each occasion saw his efforts blocked by desperate Oldham defending.
Ryan Smith was also denied by a last-ditch tackle from Neal Eardley and Karacan drilled a 20-yarder that keeper Richard O'Donnell saved smartly as Millwall huffed and puffed without making the breakthrough before half-time.
So poor during this period were error-ridden Oldham that when Constantine won a header seconds before the break and found a team-mate, ironic cheers rung around the ground.
The pattern of play in the first quarter of an hour after the restart was pretty similar to the previous 45 minutes with Millwall looking comfortable in possession, but lacking in the final third.
Smith sent a ball fizzing across goal on 52 minutes just inches away from Simpson and then Bakayogo found Karacan, whose shot hit Livermore for a corner.
But as the half progressed it was Oldham who began to ask questions of the visiting defence, and Constantine should have done better on the hour when he nipped in at the far post, but was denied by a brilliant Paul Robinson block.
Luck was certainly smiling on the Millwall goal after 68 minutes when Zak Whitbread and then Robinson were unable to get enough purchase to clear properly, and the ball fell invitingly for Mark Allott, who contrived to fire over from just 10 yards.
Ironically it was during Oldham's best spell of the game that a quick break and one-two between Sav age and Smith ended with the former's angled left-foot angled effort bobbling into the back of the net to put Millwall 1-0 ahead.
However the celebrations from the die-hard band of 220 away fans had barely died down when Taylor beat Bakayogo on the right and his deep delivery was headed home by Constantine from close-range.
Jackett promptly brought on Marcus Bignot to try and tighten things up at the back, although it took a great goalline clearance by Ali Fuseini and then a wayward half-volley from Deane Smalley in injury-time to ensure that Millwall headed back down south with a point in the bag.

Oldham Athletic: O'Donnell, Eardley, Thompson (Jarrett, 8mins (Wolfenden, 79mins)), Stam, Lomax, McDonald, Allott, Livermore, Smalley, Taylor, Constantine. Subs not used: Ollerenshaw, Black, Kelly.
Scorer: Constantine (80mins)

Millwall: Evans, Senda, Bakayogo (Bignot, 83mins), Fuseini, Robinson, Whitbread, Karacan (Cochrane, 90+1mins), Smith, Savage, Simpson, Brkovic (Harris, 81mins). Subs not used: Pidgeley, Martin.
Booking: Evans (85mins)
Scorer: Savage (77mins)

Ref: M. Jones
Att: 4,391 (away 220)