I think you should check or get it checked by electrician if the wire have the current to the oil pressure switch in ON or not or is it not getting short or body, in low gear with little bit or vibration the wire is not disconnecting from body and operating as normal, if ok should check the oil pressure sensor.
Light should be bright at ON position and turn off at engine start with in few seconds, if there is no problem. Also try to scan for codes try using the shorting method. If ECM is malfunctioning light could also behave like that I suppose