If we approach the situation as a symptom-cause situation, the symptom is
the OP light.  The cause is unknown.  There is a 3 pin voltage regulator in
the instrument pod that may be on it's way out.  However for a quick check
have the dealer troublshoot the sensor system, not the OP 'problem'.  As a
very simple check make sure the wiring and contacts from the OP sensor to
the instrument pod are intact and secondarily, measure the sender itself.
Left as an excercise for the HQOPDT (highly qualified over paid dealer
technician) to figure out how to do that.  The motor should have 20-50 oil
and an acceptable Mahle filter.

As a last resort temporarily exclude the VW measurement system by attaching
a known good oil pressure guage and reading the values at 1200RPM and
3000RPM.

Actually I would do this first but then I have a VDO pressure guage and
perhaps the service establishment doesn't.

Sometimes just removing and reconnecting the wiring will make the symptom
vanish.  A recent event at a well known shop caused considerable concern
when after some servicing, the check engine light came on periodically, the
code thrown by the ECU (SVX motor) was non sticky and showed 'ignition
system fault' indicating that the measured voltage at the ignition switch
was out of spec.  Removing and replacing the connectors to the switch module
fixed it.  The fault was indistinguishable by the ECU from an ignition fault
as rightly it should be.  However the ECU considers only it's measured
values, an open wire can give a false reading.