There are many synonyms for the phrase "be obligation". Some of the most common synonyms include "be duty", "be responsibility", "be burden", "be commitment", and "be requirement". Other synonyms that may be used include "be mandate", "be accountability", and "be charge". Each of these words has a slightly different connotation, but they all relate to the same idea of something that one is required to do. Whether it is a legal or moral obligation, the synonyms for be obligation can help convey the seriousness and importance of fulfilling one's duties.