The principal party opposed to the governing party in the British Parliament (NSOED).
"I must say, you're taking it a lot better than your predecessor. He tried to throw me out the window, thought I was a hoax planned by the opposition." (HBP1)
An exclamation, usually used to attract attention (as in "Oy, come and look at this!"). Derived from "Oyez", which is used by town criers to call attention. The origin is the Old French for 'hear'.