Where is the oil pressure sensor on a 2001 Oldsmobile Aurora with a 3.5 V-6

one and the same

On the right side of the vehicle is the oil filter box next to the filter is the oil pressure sensor.---------------------------------------------------------------------------------------
Lexia-3 V47

Not sure what "exp 18" means...but the Low Tire Pressure or tire pressure monitoring on the Aurora is done via the wheel speed sensors. Anytime you fill or add air to the tire you will need to reset the tire pressure monitoring - not just the current indicator, but go to gage->tire pressure and reset.
If any of the wheel speed sensors are having issue, this can effect the reporting to the tire pressure monitoring. If you have an ODB-II scanner that can read ABS codes this is very helpful as it will tell you exactly which sensors are having problems.

I have a 2001 aurora, if it is a 4.0 it needs 7 quarts, a 3.5 is 6 quarts.

Classic 2001 Aurora problems!1) I have found the oil pressure light comes on intermittently as well. Check the oil quality and level. On mine I have not needed to do anything else.
2) If your willing, this is a relatively inexpensive repair...http://aurorah.proboards.com/index.cgi?board=nextgeneration&action=display&thread=5353&page=2

right near the oil filter

This Indicates POOR OIL PRESSURE and PROBABLE OIL PUMP FAILURE, DO NOT DRIVE Before Correcting FIRST Have the Pressure Checked MANUALY IF OK SENSOR is BAD IF PRESSURE is NOT OK OIL PUMP has FAILED and Requires Replacment.

1) If you have any engine codes, these may be a clue. 2) Check air supply, clean filter, not blocked? 3) Check fuel pressure. 4) Crankshaft position sensor, but this will throw a code, so you should have seen this from step 1. 5) Spark plugs & ignition modules - should also throw codes. 6) Fuel injectors

Sounds like your oil pump pressure relief valve is stuck ?

The Aurora is a low pressure oil system. The only solution available at this time is to increase the oil weight. I went o straight 30W to solve the idle alarm issue. Some use 20W oil and that works too.
With 20W you will be at 80 to 90psi cold then drop to 20-30psi hot.
I first replaced the oil pressure gauge and it was worse. I put the original back in and it was better. Then went to 30W and all is fine.
Running it that way for a few years now.