Oil pressure light

hello guys , my 09 plate xf , had a coolant problem ,so I changed water pump & the plastic housing that sits top /central of engine ,,just before I drove it in for repair work ,the oil pressure light came on , so I topped up the oil as it said it needed one litre on the system ,,now it doesn't say to top up oil ,but oil pressure light is still on ,even after leaving it 30mins i restarted engine & it's still on ,,help please