It's most likely the o-ring.
If it's not, you're not going to like the alternatives.
The other two possible culprits are either the front or the rear (main) OPR's.
This is highly unlikely if the engine has never been opened up.
It's is very unlikely either would fail, but there is always a first.
I've had both of them fall out (don't ask), and they exhibit the same symptoms you describe.
If all of that checks out, you got a massive blockage somewhere in the oil system...again, highly unlikely but you never know...
-Ted