The phrase "go off rocker" is slang for someone who is behaving in a crazy or irrational way. Antonyms are words that have opposite meanings. Therefore, some antonyms for the phrase "go off rocker" could be "keep it together," "remain sane," and "stay in control." Other antonyms could include expressions like "be level-headed," "keep one's composure," and "remain grounded." These words all suggest that someone is acting in a rational, logical, and stable manner. So, if you were to use antonyms to describe behavior opposite to "go off rocker," you would be describing someone who is calm, collected, and reasonable.