Notation and shorthand of cobordism G=O, SO, U

Question

This is really a basic question:

From Wikipedia: "The basic examples are G = O for unoriented cobordism, G = SO for oriented cobordism, and G = U for complex cobordism using stably complex manifolds."

Question: I just wonder what it the meaning of abbreviation/shorthand O/SO/U which implies unoriented/oriented/complex cobordism?

I suppose O has something to do with oriented, and U is related to a complex ; what is S? What is O and U really?