Which insignia indicates the rank of O-4 in the Marine Corps?

In the United States Marine Corps, the rank of O-4 is designated as Major. The insignia that represents this rank is one gold oak leaf. The use of the oak leaf is a traditional symbol used in many military branches to denote a certain level of authority and experience, and the gold color specifically signifies the rank of Major.

Understanding insignias is crucial for recognizing the hierarchy within the military, as each rank has its own unique representation. For O-4, the gold oak leaf stands out distinctly amongst other ranks, which helps in quickly identifying the rank of personnel in uniforms and during official functions.