An emoticon intended to appear as a blushing person, the 'O's being eyes and the slashes (///) being the blush, similar to how it's represented in animes and cartoons.
It can mean someone is
flustered,
embarrassed, and more things
associated with blushing.
This emotion can also be shown as O\\\O, O\\O, O//O, O//o, O\\o, and more similar variants.
Person A: Do you have a crush on him!?
Person B: Ehmm. O//O
Person A: I'm
pretty sure Jason heard about what you did before you sobered up at
the party last night.
Person B: Oh god. O///O. Kill meee