O-3 is a military rank designation used in the United States Armed Forces, specifically for officers. In the Army, Air Force, and Marine Corps, an O-3 is typically referred to as a Captain, while in the Navy and Coast Guard, it is known as a Lieutenant. This rank is above O-2 and below O-4, indicating a level of experience and responsibility in leadership roles. Officers at the O-3 level often lead teams, manage operations, and are involved in strategic planning. They may also serve as mentors to junior officers and enlisted personnel, contributing to the overall effectiveness of their unit. The rank of O-3 is crucial for career progression within the military structure.