oil light comes on with low oil pressure.

Never heard of a oil pressure sensor failing and the oil pump is very reliable in Subarus.
Do you have any oil leaks like around the turbo banjo bolts? Id get the oil pump checked/replaced and also have a look at the sump oil pickup.

Oil pressure sensor is located on the top of the motor very close to the front. Its just behind the alternator.

Dont get on boost until you can get this sorted. I personally wouldnt even drive the car at all. You risk damaging your crank bearings and then requiring a rebuild/new short motor.

__________________
RS Liberty - Because WRX's are only good for parts...