The OKC-3S is a bayonet developed by the United States Marine Corps to replace the M7 bayonet and M9 bayonet as its service bayonet for the M16 family of rifles and M4 series carbine. This multipurpose bayonet provides greater durability than the M7 bayonet and it also functions as a fighting knife.

A bayonet is a knife, dagger, sword, or spike-shaped weapon designed to fit on the end of the muzzle of a rifle, carbine, musket or similar firearm, allowing it to be used as a spear-like weapon. The term is traditionally derived from Bayonne, the town in south-west France where bayonets were supposedly first used by Basques in the 17th century. From the early 17th to the early 20th century, it was a melee weapon used by infantry for offensive and/or defensive tactics. Today, it is considered an ancillary weapon or weapon of last resort, although it is still used for ceremonial purposes.