WordNet sense 1 (a hard brittle blue-grey or blue-black metallic element that is one of the platinum metals; the heaviest metal known):

osmium

WordNet sense 2 ((computer science) software that controls the execution of computer programs and may provide various services):

operating system

WordNet sense 3 (the left eye):

oculus sinister

WordNet sense 4 (rigid connective tissue that makes up the skeleton of vertebrates):

bone