Oil pressure will vary according to temperature, bearing wear, and the viscosity of the oil. A good-condition engine running 10W-30 oil will probably start out cold at around 40-50 PSI and drop to 20 PSI or so at hot idle. The switch that turns on the idiot light oil pressure warning contacts at about 7 PSI, so a lot of people who don't run an accurate gauge have no clue how low the pressure actually is at times. "The light's out, it must be OK, right?"
Actually, it's theoretically possible to lubricate an engine with zero oil pressure- - - -as long as all the bearings receive oil at exactly the rate it's being leaked away back into the crankcase, the pressure on the gauge will be zero. Any oil pressure at all indicates the engine is getting more oil than it needs.
Jerry
