oc. Ocean.
O.C.R. Order of Corporate Reunion.
Oct. October.
O.D. Ordnance Data.
O.E. Old English.
O.F. Odd Fellow; Old French.
off. Official.
O.H.G. Old High German.
O.H.M.S. On His (or Her) Majesty's Service.
O.K. All correct (prob. a humorous spelling of this).