Israel international Cohen has yet to feature for the Trotters this season because of a thigh injury, while O'Brien has not featured since the game against Blackburn in the middle of October with a similar problem.
Megson is hopeful Cohen will come into contention for a first-team place soon but the club will take their time over O'Brien in a bid to get him fully fit and able to enjoy a prolonged run in the side.
"Tamir Cohen is still struggling with his thigh and Joey O'Brien is the same," Megson said on the club's website.
"We are just giving Joey time so hopefully when he does come back then he stays back. He's got to build his fitness up to the highest level that he possibly can, but he's been a bit unfortunate throughout his career that he's picked up a lot of injuries.
"Tamir is due back any time but again he's been a little stop start. We could do with him having a decent run at it so that we can see what he can do."