It could be oil pressure or it could be a bad sender.  I would get a pressure
guage so you can monitor the pressure so you can see what the pressure is when
the alarm goes off.  I did this on my Vanagon.  It was pretty cheap/easy.
How To:
I bought a generic oil pressure guage from FLAPS and then I went to Napa to
get 20' of plastic oil line (I used at least 15 feet!).  I set up a "T" using
a 2 1/2" piece of threaded pipe and a "T" fitting.  The pipe and "T" fitting
are heater parts and can be got from any local heater place.  I wrapped all my
threaded ends with teflon tape (if you use teflon tape becareful not to wrap
the first couple of threads as the threads could cut up the teflon into little
pieces and these pieces could cause problems inside the motor).  I threaded
the pipe into the sender hole (the one of the bottom of the motor between
cylinder #3 and #4) and then threaded the "T" fitting onto the pipe.  In the
two other sides of the "T" fitting I put the sender and the oil line.  I did
all this with no cutting, drilling.  Then I ran the oil pipe up the bottom of
the van and tie wrapped it up all the way to the front.  There I followed the
speedometer cable right up into the dash.  Then I ran the line out through the
steering column plastic cover.  I didn't want to do any drilling, so I
attached the oil pressure guage to the plastic steering column cover with a
tie wrap.  Now I can look down through the steering wheel and check my oil
pressure any time I want.
Hope this helps somebody.
Ken Wilford
Van-Again
John 3:16
PS I offer these instructions with the usual disclaimer that if you mess up
your van its not my fault :)