Yes, OMNIBUSSES is a Scrabble word!

Letter values in Scrabble US: O 1, M 3, N 1, I 1, B 3, U 1, S 1, S 1, E 1, S 1.

Is it a valid word?

OMNIBUSSES is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

an anthology of articles on a related subject or an anthology of the works of a single author
a vehicle carrying many passengers; used for public transport

Also: double-decker, passenger vehicle.

Not playable in Scrabble US: MOTORCOACH.