Poseidon Oil was a subsidiary of Poseidon Energy before the War.
Easily the most recognizable of the companies operating under the Poseidon name, Poseidon Oil did not limit itself only to the excavation and pumping of petroleum - it also operated a number of nuclear power plants, one of which was the Reactor No. 5 in Gecko. Major installations belonging to this corporation were the Oil Rig, eventually used as the headquarters of the Enclave on the Pacific Ocean, and the Navarro refinery on the Pacific coast of California.
Together with the Enclave, Poseidon Oil helped finance Senator Todd Peterson's secret bunker. In addition, the majority of Poseidon Oil locations seemed to be specifically designed to be easily retrofitted with Enclave technology and personnel, with the notable exception of the Oil Rig's foyer floor.