A person whether it be male, female, or whatever who will only engage in sexual intercourse or activity when it takes place during an Orgy, usually involving a minimum of 5 distinct persons (animals not included).
Robert: Man, I spent three hours talking up Helen last night and buying her drinks thinking I was going to score, but wouldn't you know it she's an OSexual and wouldn't go back to my place unless there were two more couples going to join us.
Shawn: Shit! I wished you would have called me as I was out with the Mrs. and our neighbors Heather and Bill. They were both loaded up on Pino and cocaine and would have been down for anything...especially cleaning Helen's butthole.