The Poseidon Energy Oil Rig was a pre-war deep sea drilling platform, built over the world's last accessible underwater oil field. The Americans won the race to estabilish a platform there, after sabotaging Chinese efforts to do so. During the nuclear storm of the Great War, the Enclave most important members and essential personnel have evacuated to this location.
It is logical to assume that this was no ordinary derrick, but rather, a disguised, powerful military base with heavy industrial facilities, top notch medical technology, completely self-sufficient, able to provide the Enclave with a highly efficient power base.
During the course of Fallout 2, the Chosen One must travel there via an oil tanker in order to save his tribe and the previous inhabitants of Vault 13. The Enclave and President Richardson were preparing to release a form of the FEV virus into the world's jet streams, thereby preparing the Earth for re-colonization and killing everyone in the process. The Chosen One kills the President and destroys the Oil Rig by deactivating/destroying the coolant systems, overloading the primary reactor and causing a megaton-sized thermonuclear explosion.
The massive structure of the oil rig probably had many facilities for industrial manufacturing, research and development, Vertibird maintenance etc. However, in-game, only a small portion of the Oil Rig is visitable.
The entrance to the rig, a large dock illuminated with yellow spotlights. Every arriving vessel docks here, and the PMV Valdez is no exception.
The entrance hall of the rig, lined with powerful auto-turrets that are designed to obliterate everything and everyone foolish enough to attempt an attack on the installation. The main hall's floor is a massive relief with the word Enclave stamped on it, with a small access terminal in the lower left corner. The hall connects via a spiral staircase to the detention/R&D level and the barracks, via a passageway with a waiting room.
The final battle with Frank Horrigan is fought here.
Connected to the entrance hall, the Barracks area housed a storeroom, where T-51b Power Armors were used instead of forklifts, troop housing (together with a large gym and lavatories), an armory stocked with explosives, the admin section for the soldiers and their CO's living quarters. A spiral staircase sealed behind flood doors connected to the Presidential level.
Presidential Security Checkpoint:
Houses the prison cells used to contain the Arroyo villagers and the people from Vault 13. Also contains several small labs and medical facilities, and storage areas. It is located south from the entry hall, and connects to the Trap Room.
A series of small rooms with computer-controlled doors and an electrified floor. Unless the right combination of computers are accessed, the doors will trap the unfortunate explorer and leave them at the mercy of the floors. Two small side rooms provide storage and a reprieve from the traps. It is south of the Detention Level and connects to the Presidential Level.
Home of the offices of President Dick Richardson, Vice-president Bird, Richardson's secretary, and the Enclave's other elite. Also contains a bomb, a war room, a conference room, and medical facilities where testing on the FEV is being done. A rear staircase connects it to the Reactor Level, and an emergency stairwell connects it to the Detention Level.
The deepest level of the rig, it houses the central computer, the rig's power generators, a small barracks, and several scientists and citizens to maintain the former two. It is only accessible by a staircase on the Presidential Level.
As a military base, the Oil Rig was heavily defended - Captain A. Ron Meyers refers to Poseidon guns, that are able to blast even a tanker-size vessel out of the water, if it doesn't have the Poseidon IFF protocol active. Likely, anti-aircraft weapons were present, in addition to a significant Vertibird fleet.
Internally, apart from Enclave soldiers outfitted with Advanced Power Armor, an extensive network of security turrets (dual 5mm miniguns) and sentry robots was present, ready to obliterate any foe. Quite probably, the Oil Rig was the single most well protected location in the entire post-nuclear America.
The Enclave Oil Rig was home to America's finest minds and people... in theory at least. In practice, years of isolation and propaganda have left the citizens of the Enclave decadent, xenophobic, and extremely dangerous to outsiders, in particular the soldiers.
Clearly identifiable were the "classes". Citizens wore blue and yellow jumpsuits, identical in design to those of Vault-Tec shelter inhabitants, and carried no weaponry. Members of the science division wore lab coats and carried personal defense weapons (usually a Wattz 1000 Laser Pistol), while the military (including the Secret Service) used T-51b Power Armors and M72 Gauss Rifles (lower-ranking soldiers) or Advanced Power Armor and various heavy duty weapons.
The President was unarmed, but carried what was probably the most important asset on the Oil Rig - the Presidential Access Card, giving him full control over the systems of the drilling platform.
If you have your Science and Speech skills high, you can try repairing computers in the room with electric floor. If you are successful you will obtain the Alien Blaster, if your Speech skill is high enough.
The Oil Rig appears at the very end of Fallout 2.