Forward Operating Base

A forward operating base (FOB) is any secured forward military position, commonly a military base, that is used to support tactical operations. They may or may not contain an airfield, hospital, or other facilities, and are generally separate from a main operating base by being temporary stations rather than permanent.

Militaires Sans Frontières
The Militaires Sans Frontières made use of various camps due to their nomadic nature, although they eventually gained a forward operating base in the form of Mother Base, which was originally a OTEC offshore research plant before it was purchased by a front company by the KGB Vladimir Zadornov, who supplied the base to them as downpayment for hiring their services. They eventually expanded it and even completely renovated it to create a hexagon-based design. However, it was later destroyed several months later by XOF and Skull Face.

Diamond Dogs
The main headquarters for the Diamond Dogs, also called Mother Base, was situated in the Seychelles, which like Mother Base before it was also an offshore plant-based facility. This eventually received extensive expansions, eventually becoming large enough to feature its own forward operating bases, also situated on offshore-plants. These bases also end up frequently attacked by rival PMCs, often stealing various equipment and personnel during the raids.

Behind the scenes
Forward operating bases were introduced by name in Metal Gear Solid: Peace Walker, although its function was closer to a Main Operating Base than a Forward Operating Base due to Miller's comments about it being their home.

In Metal Gear Solid V: The Phantom Pain, the Mother Base gameplay mechanic was expanded to also feature forward operating bases for online mode, which would activate once Mother Base's development reaches a certain size. This also leads to these FOBs being attacked in a versus mode.