A silver oak leaf is the rank insignia for officers in the U.S. military in the O-5 paygrade, which is designated Lieutenant Colonel in the Army, Air Force, and Marines and Commander in the Navy and Coast Guard. (To avoid confusion, different abbreviations are used for Lieutenant Colonel: LTC for Army, LtCol for Marines, and Lt Col for the Air Force.) Both the tie tack and tie clasp are authorized for wear in all five services.
The oak leaf was first used as an insignia of grade on epaulettes worn by Army Lieutenant Colonels and Majors at a time (1830s) when the color of the oak leaf depended upon the branch of service: Lieutenant Colonels in the Infantry wore silver oak leaves, all others wore gold. This was because the color of the insignia had to match the color of the border of the epaulette, a system the Army eliminated in 1851 by mandating only gold borders on epaulettes and that Majors would wear gold oak leaves and Lieutenant Colonels would wear silver.
In the Navy, the rank of Commander can be traced back to the British Navy in the 1670s, where the rank immediately below Captain was called %u201CMaster and Commander;" the British eventually created several grades of the rank of Captain. Our Navy, borrowing from the British model, incorporated the second grade of Master Commandant, which became Commander in 1838 (Lieutenant Commander was established in 1862).