XOF was a military division formed before or around the 1970s, acting as the main antagonist of Metal Gear Solid: Ground Zeroes and possibly Metal Gear Solid V: The Phantom Pain. The group is made to specifically be an inverted version of FOX, a CIA subdivision in which Zero and Big Boss formerly served in.
History
XOF was involved with the capture of Chico as well as the imprisonment, torture, and likely rape Paz Ortega Andrade, a former Patriots agent. Skull Face, XOF's commander, planned it so Big Boss would stray from Motherbase, his military's headquarters, to save his aforementioned friends. After doing so, Big Boss was informed that during his rescue, XOF infantry attacked and destroyed Motherbase, killing several MSF members in the progress.
XOF also implanted two bombs in Paz, one in her chest and the other seemingly in her uterus, ultimately leading to her death and the wounding of several MSF operatives, including Big Boss, Chico, and Kaz Miller.