Yes, OPPOSITIONS is a Scrabble word!

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

Is it a valid word?

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

the action of opposing something that you disapprove or disagree with
the relation between opposed entities
the act of hostile groups opposing each other
a contestant that you are matched against
a body of people united in opposing something
a direction opposite to another
an armed adversary (especially a member of an opposing military force)
the major political party opposed to the party in office and prepared to replace it if elected