os (Noun) — (computing) software that controls the execution of computer programs and may provide various services.
os (Noun) — The left eye.
os (Noun) — A hard brittle blue-grey or blue-black metallic element that is one of the platinum metals; the heaviest metal known.
os (Noun) — Rigid connective tissue that makes up the skeleton of vertebrates.
os (Noun) — A mouth or mouthlike opening.
os (Noun) — The 15th letter of the Roman alphabet.