Hotshot Bobby Zamora was rested to the bench and then sent off as Albion crashed to a disastrous third away defeat out of four.
The drained 25-goal star replaced Daniel Webb after 56 minutes with the Seagulls 1-0 down.
Zamora made an instant impression, heading against the bar with his first touch.
But he made the kind of impact boss Peter Taylor had not been banking on when he was sent off on 84 minutes for kicking out off-the-ball at Oldham defender Chris Armstrong.
Zamora will now get the rest Taylor felt he needed in the shape of a probable three-match ban for violent conduct.
That would rule Zamora out of the home game against Wrexham on February 23, the visit to Bournemouth three days later and the trip to promotion rivals Stoke on March 1.
A match watched by a crowd of 6,951 at Boundary Park was dominated by a strong wind blowing straight down the pitch.
Albion played against it in the first half and Oldham were deservedly ahead at the break.
Midfielder Matt Appleby, on loan from Barnsley, volleyed his second goal for the Latics on 30 minutes from a David Eyres corner.
Skipper Paul Rogers dragged wide a great chance to equalise shortly before the interval when he was put through by Robbie Pethick.
Albion were much more of an attacking force with the wind in their favour in the second half, especially once Zamora came on.
He thumped a header against the bar from a Paul Watson cross and then set up Gary Hart, who was foiled by Paul Rachubka's fine smothering save.
Eyres, Oldham's topscorer, clinched their sixth League win out of ten on 81 minutes with a thunderbolt free-kick from the edge of the area.
Zamora's moment of madness shortly after may prove even more costly for Taylor's promotion chasers.
Webb also faces a two-match suspension from February 23 following his tenth caution of the campaign in the first half.
New signing Junior Lewis made his debut as a 74th minute substitute for Albion, who remain second after Bristol City were beaten 2-1 at Wycombe Wanderers.
Monday's Withdean visitors Reading are seven points clear of the Seagulls from a match more following a 2-1 home win against Notts County.
Albion will lose second place tomorrow if Stoke pick up at least a point in the Potteries Derby at home to Port Vale.
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