I'm guessing you're asking “What does OS stand for?”. The answer is Operating System. They're a very common thing people make on Scratch. Of course, these aren't completely real (They can't run real programs) like Windows 7 or OS X are, but they're still pretty fun!

OS stands for operating system.
Here is an example:
Click Me

when gf clicked
if <(mrn0body) = [awesome]>
say [yes I am] for (3) secs
else
say [No, I'm awesome] for (3) secs
end