Ford Fiesta: What could be leaking oil on the opposite side to the exhaust next to the rear left tyre?

There's a huge oil leak coming from my 01 fiesta on the opposite side to the exhaust, it's really badly rusted, does anyone know what part of the car this is? Or send me a link to a webpage that will tell me?

If you were to post a pic of leak section? Better answers would follow,
Other side of exhaust on engine is intake section. But oil can leak in one area and be blown or carried by gravity to another.
We need to clean ara first then start checking for leaks.

There's no "oil" at the rear of a Fiesta - only fuel.
The fuel filler is usually on the opposite side of the car to the exhaust tailpipe, so I suspect that you have a leak from the fuel filler neck (the pipe that leads from the fuel filler cap to the fuel tank).
If it is a petrol car, then this is a fire risk. If it is a diesel car, then you are spilling a greasy liquid right by one of your tyres, so risking a loss of control of the car when cornering. Either way you should stop driving the car (and preferably have the fuel tank drained) until you get it repaired.