To add to the S/O debate, in my airline, S/O is someone with less than 1500 hours, he/she then qualifies as an F/O. They both handle the aircraft in the right hand seat. 90% of our flying is two crew however we do some long haul sectors which warrant a third crew member and this can be a Captain and 2 F/O, 2 S/O etc...
I know Qantas and ANZ have S/O which is a job position rather than a rank and they only sit at the controls in the cruise.