OOS can happen on the decoding side usually when the Video is at too high a bitrate for the device to handle....
The Audio takes nothing to decode but the higher the bitrate the longer it takes to decode and display the video.
Therefore the Audio comes ON TIME but the video gets delayed leaving you out of sync.
Does the audio PRECEDE the Video? (this is the most common form of OOS you see on the backend)
It usually only happens on the Encoding side when the Audio and Video take two different paths to the encoder.
Thats about the only time you might see Audio delayed compared to Video.
It is possible that the decoding device can get lost or face some memory condition that would cause this but usually it's because it attempted to decode some high Bitrate video in the first place.