Yes, OPPOSITES is a Scrabble word!

Letter values in Scrabble US: O 1, P 3, P 3, O 1, S 1, I 1, T 1, E 1, S 1.

Is it a valid word?

OPPOSITES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a word that expresses a meaning opposed to the meaning of another word, in which case the two words are antonyms of each other
a relation of direct opposition
a contestant that you are matched against
something inverted in sequence or character or effect

Also: opposite word.