Oldham 2 Leyton O 0.
The Latics took the lead when Deane Smalley found Chris Taylor on the edge of the area and Taylor's clinical finish gave the home side the advantage.
Oldham continued to dominate and Matt Wolfenden and Neil Eardley went close to doubling the advantage before the goal finally came three minutes before the break.
Orient failed to deal with Andy Liddell's corner and Mark Allott let fly with an explosive shot.
Keeper Glenn Moris rescued Orient in the 58th minute with a superb stop to deny the busy Smalley.