OFFICE—charge, function, place.
OFFSPRING—issue, progeny, children, posterity.
OLD—aged, superannuated, ancient, antique, antiquated, obsolete, old-fashioned. (Young, new.)
OMEN—presage, prognostic.
OPAQUE—dark. (Bright, transparent.)
OPEN—candid, unreserved, clear, fair. (Hidden.)
OPINION—notion, view, judgment, sentiment.
OPINIONATED—conceited, egotistical. (Modest.)
OPPOSE—resist, withstand, thwart. (Give way.)
OPTION—choice.