oil tanker vs tank ship

oil tanker and tank ship both are nouns.

Word	Noun	Adjective	Verb	Adverb
oil tanker	Yes	No	No	No
tank ship	Yes	No	No	No

As nouns, oil tanker and tank ship are synonyms defined as:

oil tanker and tank ship: a cargo ship designed to carry crude oil in bulk

Other synonyms of oil tanker include oiler, tanker.

oil tanker (noun)	tank ship (noun)
a cargo ship designed to carry crude oil in bulk	a cargo ship designed to carry crude oil in bulk