I'd say chain ring is ovalised as it seems you have a tight and even tightererer spot, find tight spot adjust chain tension to suit said spot and forget about it.
Really? I thought Omniums were supposed to be pretty good in this respect. I might get it up on a lathe and do some measuring.
From my personal experience you almost nearly have a tight spot. Just ride it out.
The ring itself could be round but just not seated exactly right on the spider so sits off centre creating a cam.
@aniki started
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I'd say chain ring is ovalised as it seems you have a tight and even tightererer spot, find tight spot adjust chain tension to suit said spot and forget about it.