odd fellow vs odd man out

odd fellow and odd man out both are nouns.

Word	Noun	Adjective	Verb	Adverb
odd fellow	Yes	No	No	No
odd man out	Yes	No	No	No

As nouns, odd fellow and odd man out are synonyms defined as:

odd fellow and odd man out: someone regarded as eccentric or crazy and standing out from a group

Other synonyms of odd fellow include kook, odd fish, queer bird, queer duck.

odd fellow (noun)	odd man out (noun)
someone regarded as eccentric or crazy and standing out from a group	someone regarded as eccentric or crazy and standing out from a group