Oil pressure should rise up to 3bar (45psi) a couple seconds after you start the car and should rise immediately upon acceleration after the engine has reached a normal operating temp of 80 degrees celsius. At idle during normal operating temp, the needle should hover around 1.5 bar on the gauge. Also, you mentioned about changing your oil. Which o-rings did you replace? You only need the copper one for the drain plug and the big rubber one for the filter canister cover. The metal one can be thrown into your "spares" bin. After you change the oil when you start the car for the first time with the fresh oil in it, it might take a few seconds for the gauge to register 3bar. Once it reaches 3bar, you can shut down the engine, wait a couple of minutes, then check the oil. Hope this helps!
'67 250SE Cabriolet
'93 300E 2.8