I think it's your charging light which is on so it could be a problem with regulator or battery (possibly even the fan belt but you would have noticed if this had snapped!).
Sometime the wiring gets mixed up so best way to check what's at fault is to remove the wire connector at the oil pressure switch on the engine (it looks like a large nut with plastic insert with a wire connector and is located on the engine block just below the distributor). If the light goes out then there is a problem with the oil pressure (could be a faulty switch), if the light stays on then you'll need to check that the generator (alternator) and regulator are charging the battery correctly, check that all the connections are clean and there are no loose wires.
Good luck, let us know how you get on.