No worries - You have fitted the O-ring correctly, it is internal, there is a recess it sits in.
It only seals around the shaft and should not be interfering with getting the circlip in place.
I dug my calipers out that are stripped down, they have patiently been waiting to be rebuilt for some time - hopefully the picture and description will help you:
Red Arrow is pointing at a beefy looking spring that is pushing the silver top hat cover towards the pads.
The 2 red lines are where there is a small gap at the bottom - this shows the spring can be compressed, pushing the cover down to allow the circlip to fit.
You need to get a socket deep enough to go over the shaft and just big enough to cover the silver top hat cover, but still allow circlip pliers in - plus another socket for the other side too
In a vice, with sockets in place push down in the green arrow direction and it should give you the clearance to fit the circlip.