Where the cable from the accelerator pedal joins the throttle butterfly near the inlet manifold, there is an adjuster that sets the rest position of the butterfly, and thus the idling speed of the engine. That adjuster is usually a screw, and is call the throttle stop. Your symptoms sound like the throttle stop has moved (screw fallen out?) When the engine slows down to less than its normal idling speed (800 rpm or so) the oil pressure will fall, and the light come on. Worth getting someone who knows their way around an engine to have a quick look, if you are not comfortable with trying yourself.