O-1

In the US Armed Forces, servicemembers pay grades and ranking are set by codes. Ranks are numbered from lowest to highest. There are several classifications, E (Enlisted), WO (Warrant Officer), CWO (Chief Warrant Officer), and O (Officer). O-1 is therefore an entry level officer rank and the lowest ranking officer, being above Enlisted and Warrant Officers and below an O-2. In the Army, this would be a Second Lieutenant and in the Navy this would be an Ensign.