The phrase "was off mark" means something is inaccurate or incorrect. The antonyms of this phrase could be "right on target" or "spot on", indicating that something is precise and accurate. Other antonyms could be "correct", "exact", or "true", all implying that the information or action in question is factual and correct. Additionally, antonyms could include words such as "reliable", "valid", or "dependable", which suggest a trustworthy source or action. In general, the antonyms for "was off mark" all convey a sense of accuracy, reliability, and correctness.