The word "O.K." is a commonly used term to signify approval or acceptance. However, there are several antonyms or opposite words that can be used instead of "O.K." to convey a different meaning. Some examples of antonyms for "O.K." are "not good," "unsatisfactory," "disappointing," "mediocre," "inadequate," "unacceptable," "terrible," "poor," and "horrible." These antonyms can be used in different situations to express disapproval, disappointment, or dissatisfaction. It is essential to understand the context and tone of the conversation to use the appropriate antonym effectively.