O-3
From Conservapedia
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-3 is therefore a junior officer rank, being above an O-2 and below an O-4. In the Army, this would be a Captain and in the Navy this would be a Lieutenant.
