Yes, OK is a valid Scrabble word!
used to express agreement or acceptance
Synonyms: all right, right, right then, right you are, very well, yes, very good, fine, agreed
satisfactory but not especially good
Synonyms: satisfactory, all right, fine, in order, acceptable, up to scratch, up to the mark, up to standard, up to par, competent
in a satisfactory manner or to a satisfactory extent
Synonyms: all right, fine, well, well enough, satisfactorily, passably, tolerably, acceptably