When someone is "off mark," it means they are wrong or inaccurate in their assumption or statement. The antonyms for this phrase could include: correct, precise, accurate, spot-on, on-target, exact, or faultless. These words all indicate a level of correctness or accuracy, that is the opposite of being off mark. Additionally, words like trustworthy, reliable, and dependable could also serve as antonyms to "off mark," as they suggest a level of confidence and trust that is lacking when someone is wrong or inaccurate. By using these antonyms, we can emphasize the importance of being correct and precise in our communication and actions.