There are a zillion cars out there today that use the oil pressure sending unit as a dual function switch, the first function to send oil pressure information to the dash gauge or idiot light and the second function being to run the fuel pump after the car starts. During the start cycle there is a fuel pump relay that takes over to run the fuel pump. After the engine is running that relay turns off and the current for the fuel pump is supplied via the oil pressure sending unit If that relay fails then the car won't start until the oil pressure is up to three or four PSI.
The logic is that in a bad accident the engine would stop, the oil pressure would go to zero and the electric fuel pump would stop pumping fuel through a possibly ruptured fuel line thus preventing a disastrous fire.
When this relay fails the engine will start after the starter motor has pumped up sufficient oil pressure to send electricity to the fuel pump and run it. When you turn off the engine the oil remains in the galleys for a short period of time, typically several minutes for an old worn engine, to a few hours for a new motor with good tight clearances. Therefore, the engine will start up again shortly after shutting it off but typically will be relative hard to start for longer time periods afterwards when the oil pump has to pump the thin warm oil back into the engine oil galleys. When the engine is cold and the oil is thicker it doesn't take too long to generate oil pressure and the car will start up relatively easily, however not as easily as it would if the fuel pump relay were working. The test is to listen carefully when turning on the ignition key to see if you hear the fuel pump running - it's in the gas tank so listen there for a humming noise. If you hear no humming but the car eventually starts and runs fine, then replace the fuel pump relay.
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