Push the neatly cut pipe in, the o-ring comes out of its groove. Try again, same happens. Try a different identical fitting, with a fresh piece of pipe, same happens again. Only way to get the pipe in while leaving the o-ring in place is to lubricate the pipe, and use a gentle circling motion (rocking the pipe in a circular fashion) until it goes in.
Dying to know which fitting?
Screwfix 88980. They are unbraded ss braided hose connectors (15x15x300mm). Oddly enough, it's only the non-valve-end which is a problem. The other end is fine. Also, that end (the problem end) has two o-rings next to each other.