Oil pressure is created when the oil pump sucks up oil from the pan and pushes it through the engine's oil passages. The oil is then routed to the crankshaft bearings, rod bearings, camshaft bearings and other moving parts that needs lubrication. The oil's viscosity makes it thick and slows the flow as it passes through the passages and bearings. The amount of pressure created depends on the bearing clearances as well as the oil's viscosity. Tighter clearances and/or heavier viscosity equal more pressure.
The engine cutout image describes the stages of the oil circulation. The red color displays the pressurised stage, and the blue color as unpressurised stage. The oil pump  sucks the oil and passes it through the filter  before circulating it through the engine's oil passages. A oil pressure control valve  routes the oil back to the oil pan if the pressure is too high (due to high RPMs). The bypass valve  will bypass the filter if the engine is at risk for oil starvation. This may happen during start up where the oil's is thick (and therefore cannot flow quickly through the filter) or when the filter is clogged due to poor maintenance. On certain models the oil filter has a built-in pressure relief valve  which bypasses the filter to stop avoid damage due to high pressure.
The filtrated oil then continues to splash jets , big-end bearings , crankshaft bearings , main distribution duct , camshaft bearings  and the turbo line  which leads to the turbocharger shaft bearing . The oil pressure warning switch  will monitor the oil pressure and active the warning light (and buzzer) in the instrument cluster if the oil pressure drops below acceptable level. This is also why the oil warning light is shown when ignition is on, but the engine is not started (no oil pressure).