Tell off means to rebuke or criticize someone for something they have done wrong. The antonyms for tell off would be to commend, praise or compliment someone for their actions. Alternatively, you could also choose to ignore the wrong that was done and let it slide. Other synonyms for tell off could be to educate, teach, guide, or advise someone about a particular issue. Approving, congratulating or complimenting are some of the opposite actions for telling off. It is important to choose the appropriate response based on the situation, as well as the impact you want to have on the person you are interacting with.