fellow worker vs co-worker

fellow worker and co-worker both are nouns.

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

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

Other synonyms of fellow worker include colleague, workfellow.

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