The Collaborative International Dictionary of English v.0.48:

oil rig \oil" rig`\, oilrig \oil"rig`\n.
   A structure and associated machinery used in drilling for oil
   or gas; it is usually in the form of a tower. Called also
   drilling rig.

   Syn: drill rig, drilling rig, oil rig.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

oil rig
    n 1: rig used in drilling for oil or gas [syn: drill rig,
         drilling rig, oilrig, oil rig]