Why does the oil pressure light come on

When i'm driving and the car is moving the light doesn't come on however when I come to a stop either at a junction or a set of lights the oil pressure light comes on and then goes off again immediatley after pulling away, i've had the oil filter and oil changed but it still does it, never owned a corsa before so don't know what's going on, only bought the car today second hand and only have 3 months warranty so need to get it sorted asap, Thanks