Zero (real name David Oh, and formerly known as "O", Major Tom, Major Zero and Cipher) was the original commander of CIA Special Forces Unit FOX (Force Operation X). After Zero became a founding member of the Patriots along with Big Boss, the two friends would later develop a bitter enmity.

Prior to creating FOX, David Oh served in the British SAS alongside The Boss. During his later career in MI6, he was referred to as "O." David ultimately joined the CIA, under the bureau of classified planning, where he gained the codename "Zero" (derived from "O").

In 1962, Zero helped Dr. Nikolai Sokolov and his family defect from the Soviet Union to America, but was forced to allow Sokolov himself to be taken back due to a secret deal made between the two superpowers during the Cuban Missile Crisis. After reuniting with The Boss, the two formed the CIA's FOX Unit much to the DCI's dismay. However, Zero's own reasons for forming FOX was to prepare himself for a second attempt at recovering Sokolov.[5]

While serving in the CIA, Zero recruited Donald Anderson and Dr. Clark into the organization as he recognized their potential, though he ended up receiving complaints three days per month relating to Anderson. Zero also formed a club within the CIA dedicated to finding UMAs, and acted as its president, with Sigint as its vice president. At some point, he also took in an orphan, gave her food and a place to live. Prior to 1964, Zero obtained a residence in Portsmouth, New Hampshire.