Pawel Abbott and Reuben Hazell scored as Brighton suffered their fifth defeat in seven home games.
Brighton keeper Michel Kuipers denied Joe Colbeck before Albion's Andrew Crofts headed narrowly over.
But Abbott broke the deadlock after the break when his 20-yard strike was deflected beyond Kuipers.
Brighton's failure to clear a free-kick was punished when Hazell finished with a side-foot finish before Crofts headed against the bar for Brighton late on.
Brighton manager Russell Slade told BBC Sussex:
"Again the performance wasn't so bad, but in the boxes, defensively, two soft goals and we've had a couple of good opportunities to score.
"The chances were there in what was always going to be a tight game. We need to have a more ruthless streak, we've got to do it right."
90:00+4:38 The final whistle goes and the game is over.
90:00+4:21 Nick Blackman is flagged offside by the assistant referee. Indirect free kick taken by Gary Dicker.
90:00+3:38 The official flags Nick Blackman offside. Free kick taken by Michel Kuipers.
90:00+2:58 Unfair challenge on Sean Gregan by Adam Virgo results in a free kick. Free kick taken by Dean Brill.
90:00+2:35 The ball is swung over by Elliott Bennett, Header from close in by Andrew Crofts hits the post.
90:00+1:56 Substitution Joe Colbeck leaves the field to be replaced by Andy Holdsworth.
89:30 Outswinging corner taken by Elliott Bennett from the right by-line played to the near post, clearance by Dean Furman.
89:00 Free kick awarded for an unfair challenge on Gavin Hoyte by Alex Marrow. Direct free kick taken by Adam El-Abd.
88:09 Dean Furman takes a shot from just outside the penalty area which clears the crossbar.
86:52 Tommy Elphick concedes a free kick for a foul on Pawel Abbott. Strike comes in from Alan Sheehan from the free kick, blocked by Reuben Hazell.
83:39 Adam Virgo takes the direct free kick.
83:39 Substitution Nick Blackman is brought on as a substitute for Keigan Parker.
83:39 Chris Taylor is caught offside.
82:25 The ball is delivered by Dean Cox, Sean Gregan makes a clearance.
81:51 The assistant referee flags for offside against Keigan Parker. Free kick taken by Adam Virgo.
80:52 Booking Pawel Abbott is shown a yellow card for time wasting.
80:21 Assist by Alan Sheehan.
80:21 GOAL - Reuben Hazell:Brighton 0 - 2 Oldham Reuben Hazell scores a headed goal from close range. Brighton 0-2 Oldham.
Correction - 79:36 Tommy Elphick gives away a free kick for an unfair challenge on Keigan Parker. The ball is crossed by Alan Sheehan,
79:36 Tommy Elphick concedes a free kick for a foul on Keigan Parker. Alan Sheehan takes the direct free kick.
77:46 The ball is delivered by Elliott Bennett, Nicky Forster takes a shot. Blocked by Sean Gregan. Corner taken right-footed by Elliott Bennett, clearance by Reuben Hazell.
76:58 Foul by Gary Dicker on Dean Furman, free kick awarded. Free kick taken by Dean Brill.
76:16 Substitution Gary Hart comes on in place of Liam Dickinson.
76:04 Joe Colbeck crosses the ball, clearance by Adam Virgo.
75:37 The ball is swung over by Dean Cox, clearance by Sean Gregan.
73:32 Free kick awarded for an unfair challenge on Chris Taylor by Gavin Hoyte. The free kick is swung in right-footed by Chris Taylor.
73:00 Liam Dickinson challenges Alex Marrow unfairly and gives away a free kick. Dean Brill takes the free kick.
72:48 The ball is swung over by Elliott Bennett, clearance made by Alex Marrow.
71:28 Adam El-Abd crosses the ball, save made by Dean Brill.
70:38 Substitution Chris Taylor replaces Danny Whitaker.
68:36 Free kick awarded for an unfair challenge on Gary Dicker by Alex Marrow. Direct free kick taken by Michel Kuipers.
67:57 Nicky Forster is caught offside. Indirect free kick taken by Dean Brill.
67:11 The referee blows for offside against Pawel Abbott. Michel Kuipers restarts play with the free kick.
66:20 Nicky Forster gives away a free kick for an unfair challenge on Dean Furman. Free kick taken by Dean Brill.
65:22 Effort from inside the six-yard box by Nicky Forster goes wide right of the target.
63:57 Gavin Hoyte sends in a cross, save made by Dean Brill.
62:08 Corner taken right-footed by Elliott Bennett to the near post, Sean Gregan makes a clearance.
61:33 Dean Furman concedes a free kick for a foul on Dean Cox.
61:25 Free kick taken by Adam El-Abd.
61:25 Booking Dean Furman is shown a yellow card.
60:06 Free kick awarded for an unfair challenge on Adam El-Abd by Dean Furman. Free kick taken by Adam El-Abd.
59:20 Free kick taken by Adam Virgo.
59:20 Substitution Alan Navarro goes off and Dean Cox comes on.
59:20 The assistant referee flags for offside against Keigan Parker.
58:34 Effort on goal by Nicky Forster from inside the area goes harmlessly over the bar.
57:45 Adam El-Abd fouled by Joe Colbeck, the ref awards a free kick. Free kick crossed right-footed by Elliott Bennett, Dean Brill makes a save.
56:29 GOAL - Pawel Abbott:Brighton 0 - 1 Oldham A goal is scored by Pawel Abbott from outside the area to the bottom right corner of the goal. Brighton 0-1 Oldham.
55:26 Direct free kick taken by Adam El-Abd.
55:26 Booking Sean Gregan goes into the book.
55:14 Sean Gregan gives away a free kick for an unfair challenge on Liam Dickinson.
54:14 Liam Dickinson is flagged offside by the assistant referee. Dean Brill restarts play with the free kick.
52:44 Inswinging corner taken by Alan Sheehan from the right by-line.
52:05 Effort on goal by Pawel Abbott from just inside the area clears the crossbar.
51:04 Liam Dickinson crosses the ball, save made by Dean Brill.
48:55 Reuben Hazell concedes a free kick for a foul on Nicky Forster. Adam El-Abd restarts play with the free kick.
48:31 The ball is crossed by Andrew Crofts, clearance made by Alex Marrow.
45:01 The game restarts for the second half.
Half Time
45:00+1:56 The match has reached half-time.
45:00+0:10 Joe Colbeck takes a shot. Blocked by Adam El-Abd.
44:04 The ball is crossed by Nicky Forster, Sean Gregan makes a clearance.
42:04 Centre by Joe Colbeck.
40:56 Corner taken by Elliott Bennett from the right by-line to the near post, clearance by Reuben Hazell.
40:11 Inswinging corner taken right-footed by Elliott Bennett from the left by-line to the near post, clearance by Pawel Abbott.
39:33 Danny Whitaker has shot on goal from just outside the area which goes wide of the right-hand upright.
38:55 Corner from the right by-line taken by Elliott Bennett, Dean Furman makes a clearance.
38:46 Elliott Bennett delivers the ball, clearance by Sean Gregan.
36:35 Corner taken by Elliott Bennett, Dean Furman makes a clearance.
36:07 Liam Dickinson takes a shot. Save by Dean Brill.
35:57 Elliott Bennett crosses the ball, Dean Furman manages to make a clearance.
34:18 Inswinging corner taken right-footed by Elliott Bennett, Tommy Elphick takes a shot from close in clearing the crossbar.
32:45 Liam Dickinson fouled by Reuben Hazell, the ref awards a free kick. Shot on goal comes in from Elliott Bennett from the free kick.
31:48 Alan Sheehan concedes a free kick for a foul on Liam Dickinson. Elliott Bennett crosses the ball from the free kick right-footed from right wing, save by Dean Brill.
30:00 Outswinging corner taken from the right by-line by Elliott Bennett, Andrew Crofts produces a header from inside the area that goes over the bar.
28:52 The ball is crossed by Liam Dickinson, save by Dean Brill.
28:09 The ball is swung over by Kelvin Lomax, clearance made by Gary Dicker.
26:48 The ball is sent over by Kelvin Lomax.
26:06 Keigan Parker crosses the ball, clearance made by Adam Virgo.
25:41 Joe Colbeck delivers the ball, save made by Michel Kuipers.
25:08 Joe Colbeck sends in a cross, save made by Michel Kuipers.
23:22 Nicky Forster takes a shot. Save by Dean Brill.
22:07 Liam Dickinson gives away a free kick for an unfair challenge on Alan Sheehan. Dean Brill takes the free kick.
21:30 Nicky Forster fouled by Alan Sheehan, the ref awards a free kick. Direct free kick taken by Alan Navarro.
21:04 The ball is crossed by Joe Colbeck.
20:27 Liam Dickinson challenges Dean Furman unfairly and gives away a free kick. Dean Brill takes the free kick.
19:10 Keigan Parker is ruled offside. Michel Kuipers restarts play with the free kick.
18:03 Inswinging corner taken by Elliott Bennett.
17:07 Liam Dickinson is ruled offside. Reuben Hazell restarts play with the free kick.
16:30 The official flags Keigan Parker offside. Michel Kuipers takes the indirect free kick.
Correction - 16:12 The ball is crossed by Dean Furman.
16:12 The ball is swung over by Dean Furman.
13:34 Liam Dickinson delivers the ball.
12:52 Free kick awarded for a foul by Liam Dickinson on Sean Gregan. Dean Brill takes the direct free kick.
11:42 The ball is delivered by Dean Furman, Gavin Hoyte gets a block in.
9:06 Free kick awarded for a foul by Liam Dickinson on Alan Sheehan. Dean Brill restarts play with the free kick.
8:08 The assistant referee flags for offside against Nicky Forster. Dean Brill takes the indirect free kick.
7:24 Kelvin Lomax delivers the ball, Pawel Abbott takes a shot. Save made by Michel Kuipers.
4:21 Corner taken left-footed by Alan Sheehan from the right by-line to the near post, clearance made by Andrew Crofts.
3:26 Corner taken by Elliott Bennett played to the near post, Alex Marrow manages to make a clearance.
3:14 The ball is delivered by Elliott Bennett, clearance made by Sean Gregan.
2:31 Keigan Parker is flagged offside by the assistant referee. Michel Kuipers takes the free kick.
2:03 Sean Gregan fouled by Liam Dickinson, the ref awards a free kick. Dean Brill takes the free kick.
1:29 The assistant referee signals for offside against Keigan Parker. Indirect free kick taken by Adam El-Abd.
0:42 Joe Colbeck delivers the ball, clearance by Gary Dicker.
0:00 The ref blows the whistle to begin the match.
Live text and data provided by The Press Association.
E-mail this to a friend 
Printable version 
Goals of the Week
Bookmark with:
What are these?