OGC is a piece of ASCII "art" that I've seen passed around a lot, generally with a little bit of sexual humor implied in that, when looked at sideways it appears to be a man holding a stick which, if you apply some gratuitous imagination, can be taken as a penis that he is apparently masturbating. Basically, it's supposed to be a bit of porn in three letters. As far as dirty jokes (If you can call it a joke. Its more like an icon.) go though, I find this one to be particularly lacking in quality. I, along with several others, hold that the little man is actually quite chaste and merely carrying a Popsicle stick or beer can as he looks inquisitively back at those who continually accuse him of being a filthy pervert and observing the creepy obsession with phalli of computer nerds everywhere.
Addition:
The phenomena seems to originate here:
http://www.theregister.co.uk/2008/04/22/ogc_logo/