The Oil Ship is an original unit produced in the Naval Shipyard for $35.
When over an oil spill the player can turn it into an Oil Rig by selecting the unit and pressing 'n' which produces $10 p/m after making the oil rig. It pays the same amount as a Power Plant. They are very effective at gaining economy, because you can build four of them at the same price you would get a Nuclear Plant ($140), but instead of only giving you +$15 p/m, it will give +$40 p/m. Due to it's low price, they can be used in naval battles as a sort of distraction for enemy units.