Oxygen tank

An oxygen tank is a specialized tank containing oxygen used primarily for breathing underwater. It is often used by the military for underwater insertions, although it can also be used for mental recuperation for elderly citizens.

Usage
Paz Ortega Andrade, real name Pacifica Ocean, used an oxygen tank while using ZEKE due to the cockpit flooding. Pacifica also was trained to use it under deep-sea diving training.

During the 1990s, FOXHOUND used oxygen tanks to train its operatives in diving and underwater infiltration

Outer Heaven and Zanzibar Land used oxygen tanks that were presumably used for work relating to waterways. Solid Snake, during the Outer Heaven Uprising and the Zanzibar Land Disturbance, made use of these during his missions when procuring them.

Solid Snake and Raiden also used oxygen tanks when doing an underwater insertion into Shadow Moses and the Big Shell during the Shadow Moses Incident and the Big Shell Incident, respectively.

Solid Snake also used an oxygen tank during the USS Missouri briefing during Liquid Ocelot's Insurrection, namely as a means of recuperating after his battle with Liquid Ocelot and his nearly being killed by Outer Haven.