What is the rank corresponding to the pay grade O-1 in the Marine Corps?

The rank corresponding to the pay grade O-1 in the Marine Corps is Second Lieutenant. This entry-level officer rank is typically held by newly commissioned officers, often straight out of Officer Candidate School or the Naval Academy. The structure of military ranks is organized into pay grades, where O-1 represents the first rank in the officer category. Understanding the hierarchy of ranks, where O-2 would be First Lieutenant and O-3 would be Captain, allows for a clear distinction of the progression within the officer ranks. All other provided ranks are higher in the hierarchy, which further reinforces that the correct and appropriate answer is Second Lieutenant as the rank at O-1.