Unscrew the inner bolt until the collar is loose and can be easily compressed. Might need a bit of persuasion with a rubber mallet to insert it in the steerer. A chopstick up the steerer can be useful to press against the lower nut if it won’t tighten.
Technically you don’t really need the o-ring it’s only there to hold the bits together.
Technically you don’t even need a headset adjuster but that’s another story.
Had this problem last weekend. I put a light smear of grease around it, loosened the bolt right off, pushed it in as far as the O ring, then just eased the O ring with a small screwdriver in as I pushed it down. I found the trick was not to push it in too far too quickly which completely traps the O ring at one point