Oil pressure problem

I have a 1996 fiat cinquecento 900cc with an oil pressure problem and I am a little worried. The oil pressure light keeps coming on at idling when the car is warmed up and so far I have replaced the oil pressure switch and the sump gasket and the oil pump but it is still doing it and I have noticed that if the car is at adle for over 20 secs when you pull off there is a big cloud of blue smoke coming out of the exhaust and I know its not the head gasket because that was done when I bought the car 6 months ago. Is my engine knackered or is there something else wrong with it?