oil rig vs drilling rig

oil rig and drilling rig both are nouns.

Word	Noun	Adjective	Verb	Adverb
oil rig	Yes	No	No	No
drilling rig	Yes	No	No	No

As nouns, oil rig and drilling rig are synonyms defined as:

Other synonyms of oil rig include drill rig, oilrig.

oil rig (noun)	drilling rig (noun)
rig used in drilling for oil or gas	rig used in drilling for oil or gas