Using oil on an oily face should reduce the amount of oil the skin produces anyway.

Skin gets greasy because it's dry. Your skin does the only thing that it can to try and redress the balance, which is over-produce sebum from the pores, giving you a shiny face and potentially blocking your pores.

It might seem counter-intuitive, but moisturising the skin will lessen sebum production, as will using non-drying ways of cleaning your face. So oil as cleanser makes perfect sense. :)

I imagine you just massage it in, then clean it off. I'd use damp cotton wool for a first removal, then toner and cotton wool to clean the rest of the residue off. My toner's just organic rose water anyway. Using an oil to clean your face would mean you'd be able to use less moisturiser too, as your skin should feel soft and comfortable once the oil was cleaned off, so only a dab of moisturiser needed.

I like the idea a lot, and a good quality bottle of olive oil is the same kind of price per 100g as your L'Oréal, Garnier, Nivea, Olay etc. cleansers. Bit of a no brainer really, as the oil is natural and not crammed with chemicals.