My 97 Buick Lesabre is losing engine oil pressure while going in reverse or idling?

check your oil level first

Have your sending unit tested

it controls the fuel pump relay

if it believes you have no oil pressure it shuts down the fuel pump

or

you have a bad engine

You have THREE problems going on. First you have a dirty THROTTLE BODY and IDLE AIR CONTROL valve. The IAC controls fuel/air mixture and ALSO your idle speed. If the idle gets LOW enough it will trigger a low oil pressure light, since oil pressure INCREASES with engine speed. THe THIRD thing is the choice of cheap engine oil, which can CLOG up your oil pressure relief valve and the oil pressure switch. A good flush and fill to a synthetic oil will remove the sludge and varnish over time that REGULAR cheap oil changes have MADE for you. THe IAC is the most vulnerable to oil sludging, and will STOP reacting to signals from your CPU. Any quicky lube can clean your T.body and IAC vavle for you for $20 or so. OR you can do iyourslef if you figure out what and where they are. The IAC is the KEY to healthy O2 sensors and clean catalytic converters. GOOD LUCK!!

I think your oil pressure is going low because your car is stalling out, nothing to do with the oil. Clean the throttle body and check for a vacuum leak.

Vacuum leak