Yes, OSTIARY is a Scrabble word!

Letter values in Scrabble US: O 1, S 1, T 1, I 1, A 1, R 1, Y 4.

Is it a valid word?

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

the lowest of the minor Holy Orders in the unreformed Western Church but now suppressed by the Roman Catholic Church
someone who guards an entrance

Also: door guard, hall porter.

Not playable in Scrabble US: OSTIARIUS.