The rank of Captain is actually O6, but it is possible to be a Captain and not be a Captain. Different sized ships have different levels of a commanding officer. Almost always a commanding officer of a ship is called, Captain, even if he isn’t.

For example, a DDG has a Captain who is a Commander. However a CG has a Captain who is a Captain. Then some Captains are called a Commodore, which was a rank for an Admiral at one time.

So, are you confused yet?

At one time anyone who was in command of a vessel in the Navy was called Captain, no matter their rank. Even those enlisted sailors who commanded river boats in Vietnam could be called Captain.