Just bought a 1993 previa yesterday. after driving it for a couple of hours the "oil level" light came on. Checked the oil under seat and under hood - under hood level is a little below full. Under seat dipstic has oil FAR above the full line -- what am I to think?

the oil is filled and checked by lifting passenger seat and hatch. The under bonnet reservoir tops up oil as required to ensure all the oil ways are filled. This ensures the top end of the engine does not run dry. If all is well then you will rarely ever top this up.
Try and determine where the oil leak is coming from and keep an eye on the oil level in the sump (under passenger chair)

I would check the oil level on the dipstick, I think it means the oil is low.

most of these models have 2 oil lights.The red one is oil pressure,if flashing or on when engine is running,it is usually serious.check oil level,but usually caused by blocked pickup screen(a common toyota fault) when services aren't done.The orange oil light is tripped by an oil level sensor on the side of the sump when the oil level is low.If the oil level is ok,note this is a common fault especially if oil changes have been missed,and/or a petrol engine oil used and sludge builds up,sometimes using an additive for flushing prior to oil change can help,but may not work on 1st oil change.

open hood

normaly that would be true, except our vans have an extra oil resivewar (sorry i cant spell) under the hood, its on the passenger side of mine(battery and air filter on driver side). What it does is dump more oil into your pan if level gets old. also cycles oil from pan to res back to pan, lessens oil change visits.

There is a hatch under the driver's and passenger's seats. The driver's seat hinges up to access the oil dipstick and throttle body. The passenger seat must be removed to access the hatch below it. Sounds difficult, but there are only 4 bolts to remove.

sounds like your brake air booster is gone you must replace with remanufactured or new one! when booster fails pedel tends to get very hard to press !

If the light is flashing there could be problems with the electrical wiring. The easy places to check are at the bottom of the oil reservoir (pass. side under the hood) and at the driver side of the oil pan under the vehicle where the wires plug into the sensor. If this doesn't resolve the problem then it isn't easy, and testing the circuit will have to be done by someone who knows their electrical "stuff." Even the pump motor for the reservoir could be out.

If the Oil Level Sensor light stays on and you have already checked to make sure the oil level read on the dipstick under the driver side seat engine compartment is reading adequate, then you have a sticking oil level sensor float. Replacing the oil level sensor is easy, but extremely expensive ($485 in 2012). You can try to clean the plastic spool that the float slides on, but the problem is usually that it has swollen or expanded (likely from heat in the oil). One could try to sand it down but this would be difficult. I have made a new part out of nylon 6/6 on my lathe and replaced the plastic spool so far with excellent results. I'm considering selling it as an aftermarket part when I see if mine works. It would be a lot less than $485 dollars from Toyota. Or possibly have people send it to me and I'll repair the sensor myself.

When you say "warning light is on", I assume you are not talking about the oil pressure gauge...
If not, here is your solution:
What you have is a very common problem: The oil LEVEL sensor has failed. Previas have 2 sensors; pressure & level of the oil in the pan...What you need to do is replace it, Toyota part number : 89491-28030.
BUT...This part is $380.45. Not cheap; Sorry but true.

I have had this problem before, the first time it was discovered that the stand that support the hood was not properly secured in the clip slot.
The second time the grill was loose. Check these before going to the mechanic.

sirmickei - New York