Anyone have any idea on why the oil light comes on in a 2000 Dodge Stratus?

It means the car has low oil pressure or a bad oil pressure sensor.

Low oil pressure is usually a bad oil pump. Not a cheap thing to fix.

A sensor is a lot cheaper.

You would need to take it to a repair shop to find out what it is.

Or you can ignore it. Use full synthetic oil to give the engine a chance at survival when the pressure is low. However, this may also cause sudden complete failure of the engine at any time.

This CSI auto crime scene investigator notes that the low mileage indicates Auntie did lots of short trips. This type of driving is very bad for the engine and exhaust system due to moisture. The engine and the oil never got hot enough to burn of the naturally occurring moisture off. The moisture and the oil formed a sludge in the internal parts. This sludge is difficult for the oil pump to pump and probably is what triggered the oil pressure light. Hopefully Auntie's car is a 4 cylinder. If it is a V6 2.7 liiter engine they are famous for gunking up if the oil is not changed every 2-3000 miles.

What to do? Don't keep driving it - you risk damaging internal engine parts. Immediately have the oil and filter changed by a reputable shop, not a Jiffy Lube and Doughnut type place or a tire store. Resist their pitch to have the engine flushed - it will only move the gunk around. Synthetic oil has higher detergent properties however is more expensive. Drive the car normally but take a road trip at highway speeds for about 100 miles. Go back and change the oil and filter again. Then change it again in about 1000 miles. Also have the PVC valve replaced. The part costs under $5.00. Try to do trips of 10 miles and over. That is the distance required to get the engine up to proper temp. and burn off the moisture. The 2.7 liter V6 in particular had severe crankcase moisture problems and Chrysler was sued over this. Short run driving made the engine self-destruct. Hopefully you have the better designed 4.cylinder. Also consider draining and refilling the coolant and having the brake system flushed. Also drain and refill the auto transmission. In all the above be certain to have the shop use the factory spec fluids! For the transmission Chrysler spec was ATF+3. Coolant for Chrysler products [Dodge is a Chrysler product] similarly has specs.

It's a stupid sensor. But it'll tell the computer that something's wrong & eventually cause more $ issues. If you know someone that has a home diagnostic machine it'll tell you which sensor it is. Easy fix if you have one but it's only $85 at a shop, & they're machine will be more detailed.
Good Luck
I just thought of something, when was the last oil change? besides needing a change your oil filter could trigger the computer/light. does the motor knock? if so put oil in it without thinking about it.