A 'Bro with three O's' is a girl/women in a group consisted of all boys/men. The girl is treated as just another 'Bro', and mostly acts like another 'Bro'. May still be recognized as a girl, but for the most part, treated as just another 'guy'.
In context: Hanging with some dudes, and my Bro with three O's.
ayon kay Bro with Three O's ika-28 ng Enero, 2011