I usually have to either gently lever them if in the frame or tap with a mallet to get the o-ring to start into the hole, even if it is lubricated.
BTW: The same silicone based grease you should have for working on brakes is also good for any other rubber parts like the o-rings.
BTW: The same silicone based grease you should have for working on brakes is also good for any other rubber parts like the o-rings.