O or Os - Orgasm, orgasms.
Off-the-clock - Activities that occur with a provider that she chooses not to charge for. (e.g. "We had lunch off-the-clock")
OTOH - On the Other Hand
Outcall - The girl comes to your place.
Out or Outed - Private, personal information on a hobbyist or provider is made public. (e.g. Someone's real name and address is posted to a public forum.)
OWO - Oral Without a condom
OWOTC - Oral Without Condom To Completion