song vs opus

song and opus both are nouns.

Word	Noun	Adjective	Verb	Adverb
song	Yes	No	No	No
opus	Yes	No	No	No

As nouns, opus is a hypernym of song; that is, opus is a word with a broader meaning than song:

Other hypernyms of song include composition, musical composition, piece, piece of music.

song (noun)	opus (noun)
the act of singing	a musical work that has been created
a short musical composition with words
the characteristic sound produced by a bird
a distinctive or characteristic sound
a very small sum