A face expressed as being in awe, or in complete shock of what he/she has learned by seeing or hearing it. A stupified expression. The @ symbol for the eyes shows that the person's bottom eyelids are raised, like as though he/she also shows a sign of being a little disturbed.
Dude 1: Hey, guess what? I made out with this one chick named Mary Hawkin. She was so good in bed too.
Dude 2: @o@
Dude 1: Impressed, huh?
Dude 2: I wonder if you realized that Mary Hawkin used to be "Harry Hawkin"? You know... a guy!?