A good first place to start your diagnosis of a low oil pressure condition is at the dipstick. Check the oil level to see that it is at the proper level (that is not low and not overfilled). If it is low, it may be that the engine may be burning oil, leaking oil or neglected. Topping oil may temporarily remedy the low oil pressure situation, but unless the oil level is properly maintained, the problem may reoccur.
If the engine is leaking oil, get new gaskets or seals to fix the leakage. If the engine is burning oil, the valve guides and seals are often likely worn out, but the rings and cylinders might be bad, too. Having a wet compression test or leak down test will indicate if the valve guides or rings and cylinders are worn. The least expensive repair in the case of worn guides would be to install new valve guide seals (where applicable) without pulling the head. However the best repair would be to pull the heads and have the guides lined, knurled, replaced or reamed for oversized valve stems. Worn rings and cylinders would need a complete overhaul.
Also check the condition of the oil and make sure it is the correct viscosity for the application. Heavier viscosity oils such as 20W-50, straight 30W and 40W may help maintain good oil pressure in hot weather, but are too thick for cold weather driving and may cause start-up lubrication problems especially in overhead cam engines. Light viscosity oils, on the other hand, such as straight 10W or 5W-20 may improve cold weather starting and lubrication, but may be too thin for hot weather driving to maintain good oil pressure. That is why most OEMs today recommend 5W-30 for year-round driving in modern engines.
If you have checked and the oil level is okay, the next thing to check should probably be the oil pressure sending unit. Disconnect the system and check if the warning lamp or gauge reading changes. If the warning light remains on with the sending unit disconnected, there is probably a short to ground in the warning lamp circuit. Likewise, if there is no change in a gauge reading the problem is in the instrumentation not the engine.
Bad oil pressure sending system are very common, so many technicians will replace the unit without checking anything else to see if that solves the problem. This step might save you some time, but it is risky because until you measure oil pressure directly with a gauge attached to the engine you have no way of knowing if pressure is within specifications or not. Most warning lamps won’t come on until oil pressure is critically low (less than 4 or 5 lbs.). So never assume the absence of a warning lamp means oil pressure is okay, especially if the engine is making any valve or bearing noise.
more;http://www.automobileglobe.com/how-to-diagnose-oil-pressure-issues/

how come this is not on front page? OP thanks for this contributions