RSO: Registered Sex Offender
SOR: Sex Offender Registry. Sometimes preceded by the two-character state designation ("CASOR" for "California Sex Offender Registry)
More pointers:
In general, use the term RSO rather than "SO." For those sex offenders who aren't registered, use a description that indicates their status. For those not convicted yet, use "alleged sex offender." For those convicted sex offenders who no longer have to register (such as an expiration of their registration period), use "former registered sex offender." For those who've never had to register because their state law precludes them from the registry, most likely for complete discharge of their legal status including parole or probation before registration laws became effective in their state, call them "convicted sex offenders." In addition, that term can be used as well for any sex offender convicted in a court of law. Finally, use the "predator" designation when appropriate in a legal sense, or colloquially, use for any reference to a sex offender.