Yes, OS is a Scrabble word!

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

Is it a valid word?

OS 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
a nonmetallic bivalent element that is normally a colorless odorless tasteless nonflammable diatomic gas; constitutes 21 percent of the atmosphere by volume; the most abundant element in the earth's crust
the 15th letter of the Roman alphabet
the blood group whose red cells carry neither the A nor B antigens

Also: atomic number 76, operating system, oculus sinister, o, atomic number 8, type o, group o.