Make sure the oil level is correct, and make sure you haven't disturbed the wire on the oil pressure switch. If both of these are right then you will need to check the oil pressure with a pressure gauge to ensure it is correct. You cannot afford to just assume it is. A loss of oil pressure will cause serious and irreparable engine damage.
Do you get the message just after the engine is started, or all the time it's running?
If it's the former, it could be that the oil filter doesn't have an anti drain valve that works. I would fit a new OEM spec filter asap if that's the case.
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