I was away for a few weeks and came back to the Engine Oil Level Sensor Fault message. When I removed the lower engine cover I discovered a rat had chewed through the oil sensor harness (and the little basket had chewed it very close to where it joins to the main harness below the air pump making it a PITA to strip what was left back for the repair). After soldering in some new wires, heat shrinking the joints and putting it all inside some new spiral wire wrap, I reconnected everything and still had the Engine Oil Level Sensor Fault message. Initially I thought that the marginally thicker wires I had spliced in maybe causing too much resistance for the car to measure the sensor readings accurately. I took a WAG and discovered that you need to disconnect the battery (and I touched the disconnected positive and negative battery cables together for a few seconds for good measure) and the message went away when I reconnected everything.
So, the moral of the story is to get a rat trap or a pet cat......

 

Yep, hard reset when replacing a sensor. Had the same problem when I replaced a coolant temperature sensor. Took a hard reset to clear the fault.