Yes, OSSA is a Scrabble word!

Letter values in Scrabble US: O 1, S 1, S 1, A 1.

Is it a valid word?

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

a mouth or mouthlike opening
a hard brittle blue-grey or blue-black metallic element that is one of the platinum metals; the heaviest metal known
(computer science) software that controls the execution of computer programs and may provide various services
the left eye
rigid connective tissue that makes up the skeleton of vertebrates

Also: atomic number 76, operating system, oculus sinister.