The "oil" warning light on my Charade xte starts.

The "oil" warning light on my charade xte starts to glow once i've been driving for 15-20mins. I've checked the oil level in the car and that's fine, as is the radiator coolant level, however there is a slight leak from the transmission fluid once the car stops. The temp gauge remains constant mid level whilst driving the car with the oil light glowing and there are no noticeable banging or clunking sounds coming from under the bonnet. Any ideas why the light may be coming on would be much appreciated. Thank you