There is absolutely no other definition for an o face besides the face a youtube viewer makes when he or she is craving an outro performed by Tobuscus.
Also known as "outro-jingle face," or "outro face"
I was totally having my o face on, and then he finally fufilled my needs.
Tobuscus: You had that outro-jingle look on your face - no offence. That's a compliment. A very attractive feature in a lot of cultures, called outro face - look it up! - or o face, for short.
In 2010, Thomas' aghast face, also known as the infamous "O" face, when the Fat Controller asks the inspector if he saw fish, along with the musical note that went with the shot, became an internet meme.