I have no advice to offer you as I have no experience with o-ring piston seals, other than that my first spring piston gun (purchased in November 1981), a BSA Meteor, utilized an o-ring piston seal. I left that rifle with a relative, in 1983, after visiting them at their farm. I revisited them again in 2010, but I didn't ask about the BSA and do not know if it was problematic for them. It was trouble free for the 2 years that I had use of it though.
I'm interested in your problem though as I fully intend to machine the parts for either one or all of my three spring piston rifles. If the first gun proves to be an improvement over the parachute type seals, then I'll be willing to turn the parts for the others, as well. I'm certain that there will be trial and error/learning curve to figure out the proper 0-ring size, to get just the right amount of compession to A) seal properly, with minimal friction and B) to avoid the O-ring from rolling, which, I think, is what you may be experiencing.
There is a thread on GTA,
http://www.gatewaytoairguns.org/GTA/index.php?PHPSESSID=ccf3dfe4c0e117a72c133bee1478a3a5&topic=43773.0 . If you scroll down the page to just before the halfway mark, the discussion begins about o-ring piston seals. I'm not a GTA forum member, but perhaps if you contacted "NCED", from GTA forum, he might be able to help you out.