A group, chums, family, a close group of friends that reminisce on an earlier time. It stands for Pre Summer O5. Any year can be thrown in there, it usually is picked by a special or traumatic event. Where you wish you were back in that time period or back in time to change the event or re-live it.
"Man you remember PSO5?"
"Things were great before the gang bang in the summer O5, all of this trouble is because we didn't invite johnny that night really has strained our relationship."
"He will never forgive us."