Oil pressure is to be understood as the resistance in the engine which the oil overcomes to establish a certain flow rate.
The most important criterion for good oil lubrication is optimal oil flow and not oil pressure. Low oil pressure is not necessarily bad, in fact it can be beneficial to some extent. High oil pressure is not necessarily good, in fact it can be a cause of concern.
Oil pressure is not built by the oil pump, but by forcing oil through small passages. Small oil passages with high viscosity oil will have low oil flow rate, whereas large oil passages and low viscosity oil will have a higher flow rate.
A fast flow rate with lower oil pressure will result in minimal wear and optimal cooling. A low flow rate with high pressure will result in a less efficient lubrication and cooling and therefore more wear on the engine.
Too high an oil pressure can result in failure of seals and leaking of oil and should therefore be considered as an indication of poor flow.
On a cold start the oil viscosity is high (oil is thick) and the oil pressure is therefore higher. As the engine warms up, the oil viscosity decreases (oil becomes thinner) and the flow rate therefore increases while the oil pressure drops and stabilizes. When the oil pressure is stabilized, the oil flow is at its optimal rate and the wear on the engine is minimal.
Under ideal circumstances, the oil pressure should stabilize soon after startup. Any sudden larger increase or decrease should be investigated.