Oldham had to settle for a point after an injury-time Leon Clarke goal was ruled out for offside at Brisbane Road.
However, it was a fine point for the Latics who fought back from 2-0 down.
The hosts went ahead in the 13th minute when Gary Alexander drifted a shot in, and Gary Hooper lobbed a second after Alexander saw red for a second booking.
Oldham reduced the deficit when Chris Taylor headed home and Andy Liddell snatched a point from the spot after Brian Saab had fouled Richie Wellens.
Leyton Orient: Garner, Miller, Mackie, Saah, Lockwood (Tann 58), Tudor (Hooper 11), Easton, Demetriou, Corden (Thelwell 82), Alexander, Jarvis.
Subs Not Used: Mulryne, Morris.
Sent Off: Alexander (49).
Booked: Alexander, Demetriou.
Goals: Alexander 13, Hooper 70.
Oldham: Pogliacomi, Charlton (Edwards 90), Haining, Gregan, Eardley, Liddell, Wellens, Rocastle, Taylor, Clarke, Warne.
Subs Not Used: Turner, Blayney, McDonald, Smalley.
Booked: Eardley, Clarke.
Goals: Taylor 73, Liddell 81 pen.
Ref: P Joslin (Nottinghamshire).