Oldham , like the valleys , like Thanet, like some of the Lancashire milltowns is poor because the industries that provided employment utterly collapsed. This destroyed the local economies , immigration is then high into these areas because they are poor and cheap. In these areas those who got good emolument moved elsewhere. It's the story of economics of capitalism and of any country in the world. Barely a city in Europe , Asia or Africa doesn't have these issues and it happened before the EU existed and will happen long after whatever the next manifestation of the EU is