^ Hang on, that's backwards.
A positive value of the A/V Sync setting ADDS Audio Delay. This is the normal use, i.e., to add delay to the audio to compensate for slow video processing in your Display.
An A/V Sync setting of +40ms will delay the audio an additional 40ms.
The default setting of 0ms means that audio and video are in sync -- aligned in time -- at the player's outputs, regardless of the combo of inputs, outputs, formats, and processing settings in use.
To accomplish that, the player has its own, built-in audio delay adjustments which match audio to the video processing time needed inside the player itself for the combo in use. This is complicated stuff, but you don't need to know the details. It is OPPO's job to do this correctly regardless of your choice of connections, formats and settings.
Audio delay is accomplished by buffering the digital audio stream. Video processing is always the thing that takes longer, so delaying audio is the only correction needed. This is a good thing, since attempting to buffer video (i.e., to delay video) would take way too much memory.
OK, so what about the NEGATIVE A/V Sync setting choices?
A negative valued A/V Sync setting REDUCES the default audio delay built into the player to match audio and video at the player's outputs. It makes audio come out earlier.
Now think about that. I just said you can't delay video, and obviously you can't make the audio play ahead of what's coming in as content. So the only way to make audio come out earlier is to reduce the DEFAULT delay the player is providing!
So a negative A/V Sync setting will make audio come out earlier only SO FAR. Once you have eliminated all of the default delay built into the player for your combo of inputs, outputs, formats, and processing settings, making the A/V Sync setting even MORE Negative will make no further difference.
The things to keep in mind are:
1: A/V Sync of 0ms is the default. Audio and Video are IN SYNC, by design, at the player's outputs.
2: A/V Sync greater than 0 adds audio delay. This is the normal adjustment to correct for uncompensated video processing time in your AVR or Display.
3: A/V Sync less than 0 reduces audio delay, but only up to the built-in delay the player needs to accomplish (1), above, according to your current usage. If you find you need to use a negative value, that usually means your AVR is mistakenly applying too much audio delay ITSELF.
Again, video processing is ALWAYS the time consuming thing, so how could the AVR screw this up? First the AVR may be set for automatic lip sync correction, and the requested delay value it is reading from your Display to accomplish that (it's really just that simple-minded) may be BOGUS.
Another common way this happens is when you use both HDMI outputs and thus bypass the AVR for video. The AVR doesn't know that's happening, and so it may be adding an unneeded chunk of audio delay all on its own -- i.e., even if you SET IT to zero added delay -- to compensate for its own (unused!) video processing time. I call this the "helpful AVR" problem. Cure that by disabling video output from the AVR, or setting it to "video pass through" so the AVR can know it is not expected to process the video.
The more normal case, where you need to ADD audio delay (A/V Sync greater than 0) may be curable by turning off unnecessary or cumbersome video processing in your Display. Common time wasters in many displays are video "enhancement" processing and "motion smoothing". Some Displays also take too long when fed 1080p/24. Set 1080p/24 Output OFF in the OPPO for those. And it is almost always best to disable video "enhancements" in Displays. These are designed to make poor quality video more tolerable and just get in the way when playing high quality video as from the OPPO.