The term 'I Do, For One' can be summarized by two crossword solutions: OATH (4 letters) and VOW (3 letters). An OATH is a solemn promise or statement of truth made in a formal setting. It is often used to signify loyalty or commitment to a cause, such as in a marriage ceremony. A VOW is a solemn promise made to a deity or person. It is usually made in a religious context and is often associated with a promise of dedication or service.