First Lieutenant (O-2) is the second-lowest ranking commissioned officer. It is one grade above the rank of
2nd Lieutenant (O-1) and one grade below a
captain (O-3). Second lieutenants are usually promoted to first lieutenant after 18 months of commissioned service.
The difference between the two ranks of Lieutenant is slight, primarily being experience and higher pay. It is not uncommon to see officers moved to positions requiring more experience sometime after promotion to first lieutenant. For example, in the Army these positions can include leading a specialty platoon, or assignment as the executive officer for a company-sized unit (65-150 soldiers)
