The phrase "do off top of head" refers to doing something without preparation or prior thought. Some antonyms for this phrase could include: plan, prepare, deliberate, calculate, ponder, contemplate, analyze, research, investigate, and strategize. These words suggest a more thoughtful and intentional approach to problem-solving or decision-making, as opposed to the spontaneous and impulsive nature of "doing off the top of one's head." While there are certainly times when quick thinking is necessary, taking the time to plan and consider all options can lead to more successful outcomes in both personal and professional endeavors.