co-worker vs fellow worker

co-worker and fellow worker both are nouns.

Word	Noun	Adjective	Verb	Adverb
co-worker	Yes	No	No	No
fellow worker	Yes	No	No	No

As nouns, co-worker and fellow worker are synonyms defined as:

Other synonyms of co-worker include colleague, workfellow.

co-worker (noun)	fellow worker (noun)
an associate that one works with	an associate that one works with