about my vw passat

i have a 1991 passat.i brought it used.i've already put $1000.00 worth of work in it.now that's it's back from the shop i'm still having problems.my oil light comes on with a beeping noise.it beeps cont.it won't go off.my mechanic said it could be my oil pressure pump but i'm not losing power.what could it be.someone please help.