The phrase "are on back of" implies a location or position at the back of something or someone. The antonyms for this phrase would be "are in front of" or "are on top of." While "are in front of" refers to a placement at the front of someone or something, "are on top of" means the opposite of being at the back or bottom, indicating a position on the top or upper part of something. Using these antonyms can help to provide clearer descriptions and directions in situations where location or position matters.