What is the highest rank indicated by O10?

The highest rank indicated by O10 is indeed the Chief of Naval Operations (CNO). The CNO serves as the principal naval advisor to the President, the Secretary of Defense, and the Secretary of the Navy, holding significant responsibilities in overseeing the naval forces and ensuring readiness. This position is typically held by a four-star admiral, which also corresponds to the O10 pay grade. While the term "admiral" can refer to various ranks, including rear admiral and vice admiral, the CNO specifically represents the highest authority among admirals in the Navy. This makes the CNO the pinnacle of naval command structure within the United States Navy.

Other ranks like captain and vice admiral fall below the O10 designation and thus do not represent the highest possible rank. Captain is an O6 rank, and vice admiral is an O9 rank, which demonstrates the hierarchical nature of naval ranks where O10 clearly stands apart as the top level of authority.