In my setup, only the video is connected to the OLED through HDMI, not the audio. I'm using the analog outputs on the Oppo for audio; I like the DACs and it simplifies my system.
If I send both video and audio to the display via HDMI, A/V sync is perfect, so the TV and player do get along well in that configuration. That's maybe why the PS3 performance is so well, both audio and video go straight to the set.
Adjusting the A/V Sync on the player does not completely solve lip-sync when I use the Oppo as my preamp. I agree with you there. It seems to drift, depending on the material. For example, with the A/V Sync set at 170 ms and supposedly correct, the audio still arrives a bit early on the Disney Wow disc. Like when I make a chapter change, the audio comes on before the video does.
I'll plug in my Onyko 818 AVR this week and test the input lag that way, and see if I need the receiver.