Hope this group can help with my son'* 99 LeSabre.

About 24 hours ago the Check Engine Oil light came on.
He mentioned it to me yesterday evening and as we backed the car out of the driveway, there was a pool of oil laying there.....it continued to drain out of the car.
I checked the dipstick and it is bone dry.....it was full less than a week ago.
There was no evidence of oil on the top of the engine but the bottom was covered/splattered just about everywhere.

My son doesnt remember bottoming out or anything.

So far I have done the following:
- Checked the Oil Pan and there are no holes in it
- Checked the Oil Filter and it is on there solidly
- Checked the Oil Drain Plug and it is on there solidly
- Checked the spark plugs and all are solidly in place
- Poured one quart of oil into the engine to see if oil would drain out anywhere and it did not.

My mechanic mentioned that the problem could be the "Rear Main Seal" and I read somewhere else that it could be the "Oil Pressure sending unit".

Does anyone have any idea what else I could try. I am not a mechanic so am hoping there is a relatively easy solution but am worried that this could get expensive.

I appreciate any and all help. I can post photos if necessary....just let me know what I should be aiming at!!

Finding it wont be to hard. First, clean everything up real good. Then park car on a leval spot, in a garage is best. Slide a big piece of clean cardboard under the engine. Add oil so it'* full. Start engine and let it run a bit while you watch. If you see oil dripping, shut off engine. Other wise let it run a bit, then shut it off and sit over night.

Now look at the cardboard for any drips. This will tell you where to look. Drivers side center, is rear main seal. Passanger side center is front seal. Passanger side rear is oil filter, filter adapter, or oil pressure sender. Front a bit right of center is the oil dip stick. Anything around the center of the engine is the oil pan gasket, drail plug, or oil level sensor.

First of all.....many thanks for your timely and detailed advice! Much appreciated! Here is what happened.
Followed your directions exactly.
As soon as the engine was started, oil started coming out in a steady stream.
I had the underside well lit and it was coming out on the passenger side, just to the side of the oil pan (I was under the car on the passenger side and it was on the side of the oil pan closest to me).
The LeSabre is as you know really close to the ground and I couldn't get under it to get a closer look...should have sent one of my skinny teenage boys under there with a camera!!
Is this location considered the passenger side center or passenger side rear?
We are trying to find a mechanic who will give us an estimate and I am trying to figure out a way to drive the car to them vs. pay for a tow truck. Any suggestions on how to patch it for long enough, so that I can drive it?

Without knowing exactly where the oil is coming from it'* hard to give a fix. A picture would help us. A steady stream would lead me to the oil filter, oil filter adapter, or oil pressure sender. The oil pump is right behind the harmonic balancer.

Well it took a week.....been buried with work but we got a good picture today. Looks like some kind of bolt or sensor right next to the oil filter. The oil streams out in 2 directions. Does this help diagnose what the problem is? I did find out that my Insurance company will pay for a tow to a mechanic....or can I replace this myself?
Thanks in advance for any and all advice.....much appreciated!

That'* your oil pressure sender. Disconnect the plug, get the proper size socket, and remove. Any parts store should have it in stock.

Oil pressure sender. Easy fix. I think they cost about $25 or so. A small amount of oil will drip out when you remove the old one.

Thanks guys for the diagnosis.....will be attempting the repair myself tomorrow. For a non-mechanical type, this will be an adventure but luckily between my virtual support team and neighbors with tools.....I should be good!

Your biggest enemy would be accessibility. It is a cakewalk from under the car.

It is also accessible through the front passenger wheel well with the wheel removed.