I've been reading up on this and am now confused!
1) Do you have an info centre on top of the dashboard, in the centre of the car?
2) Is this where the message appears?
3) Is the oil pressure warning lamp in the rev counter?
4) Is this the one that does not come on at key-on?
5) Are you saying that the message is not displayed on the info centre when instructed by mes, and the warning lamp in the rev counter does not light?
Assuming answers are all yes: as far as I can tell from the limited info I have, the rev counter warning light is the only connection to the sensor. The rev counter passes the information to the speedo, which passes it on to the info centre via a serial link.
This would suggest that you have a short to earth on the warning lamp circuit inside the rev counter, that is making it think the sensor switch is closed (low pressure). This would also stop the warning lamp lighting.
Unfortunately, the spurious messages the info centre is giving at key-on, and the failure to respond to instructions, make it look like it is faulty as well, or the link(s) to it are. If this is the case, then the warning lamp failure in the rev counter might be a simple bad connection between it and the sensor.
All of which is a long way of saying that this is a tough one to diagnose remotely. As well as kandlbarrat's suggestions you could clean the connectors on the rev counter, speedo and info centre, and try disconnecting the battery for a few minutes.
Questioning whether this is an mot failure might be more effective...
1.I have an info centre between the speedo and the revcounter and below the fuel and temp guages
2.The message appears here saying "low oil pressure" only when the engine is running.
3.As far as I can see there is no oil pressure warning lamp, just this message.
4. At key on the car goes through its checks of oil level bars, etc but only on starting the engine does the low oil pressure message appear.
5. Yes when connected to ECUSCAN via the ecu port in the foot well, I instruct the oil pressure message to cycle and the software says test ok and finished but there is no message appearing on the dash. Others do appear when instructed to cycle eg brake pad wear etc
I also disconnected the engine sensor today and ran engine same message readout, I then earthed the lead to the sensor same message appeared,
I checked voltage to sensor it is 0.5V when ignition off, 12 volts when ignition on, and 12.5 volts when running.
As for the MOT I think it was they wouldn't rev the engine to test the emissions even though I assured them it was just the sensor not really a low pressure.
thanks very much for everyone taking the trouble to reply and your help, I hope my answers have clarified the problem a bit more.