dont assume the worst just yet mate, the only reason it woul stay constantly on is if it is shorted, the circuit runs on a positive line constantly and the oil pressure switch creates the circuit by earthing it when the pressure gets too low, check that the wire has not come off the sensor and is not stuck to the chassis somewhere coz this is creating the full circuit constantly, if not then either a dead switch or you do in fact have oil pressure lower than 5 to 9 psi, or something like that, not good if this is the case
owen