What does OC means in buying a new graphics card?

[Cuboxx]

So I wanna buy a new Graphics card, I saw the same graphics card with just 2 letters different, OC, so, is it already overclock or it can be overclock,

Help me, I need answers

 

[Higgins4272]

It's factory over clocked.

 

[Isokolon]

It is already overclocked.
Depending on the card this provides an Edge or is just marketing