What element has the symbol 'Os'?

The story behind the answer

The symbol Os stands for Osmium, element 76. The other options are not elemental names: “Osmaridium” (more correctly osmiridium) is a natural alloy of osmium and iridium, while “Osmiumo” and “Osmalite” aren’t elements. So Osmium is the only correct match for Os.

Osmium is a bluish-gray, extremely dense member of the platinum group—often cited as the densest naturally occurring element. Its name comes from the Greek osme, meaning “smell,” because osmium forms osmium tetroxide, a volatile, pungent, and highly toxic compound. Memory tip: think “OS = Operating System = heavy-duty,” and Osmium is the heaviest-duty metal by density. Or remember “Os-me” for “odor-smell” (osme), linking Os to Osmium’s smelly tetroxide.