On a Diesel engine there are a whole number of things that have been mentioned by others which incidently are equaly applicable to Petrol engines. There is also another two things that can cause low oil pressure one of these unique to Diesels. Lets go through all of them including things already mentioned by others:
Wrong grade of lubricant.
Aged oil - not changed frequently and contaminated.
Worn bearings (mains, bigends, camshaft bearings). The first two are likely to cause noise.
Worn oil pump.
Broken spring or jambed open relief valve and or not seating.
Leaking seal on the ouput side of the turbocharger but would be obvious with coloured exhaust black or blue but blue mostly.
Internal oil leak from say a stuck open filter pressure relief or an oil filter that is off its seat because either the wrong one or it is blocked and lifted off its seat into bypass mode.
Oil dilution by diesel fuel. Part a. Many diesel engines have the injectors within the rocker cover and if the leak down pipes (old fashioned mechanical injection) or fuel rail (CRDI) are leaking the fuel internaly into the engines crankcase via the oil return passages to the sump the crankcase oil will become diluted and oil pressure will drop. Part b. The metering pump (mechanical injection) or the high pressure pump (CRDI) can have an input seal fail and diesel ends up in the sump via the timing case. On an engine with an inline plunger metering pump and if driven by an external coupling this is unlikely.
Rare faults - an engine with an internal main oil gallery plug or a casting fault on the oil gallery leaks straight back into the sump. Some diesel engines with two oil pressure relief valves eg Nuffield tractors, the rocker gear relief valve may jamb open which is pretty rare since the usual mode of failure on these was to jamb shut and you had very high oil pressure but you new about it as the pressure gauge bourdon tube would rupture and the gauge dial would go black or the glass would break as well and your legs would be doused with hot black oil.
Everything is OK but the gauge showing the oil pressure is faulty.