FD and
FFF are correct, a missing oil filler cap will make no difference to your oil pressure. The engine crankcase has a breather to allow any accumulated pressure build up from combustion gasses passing past the piston rings and valves to vent to atmosphere. If you left the oil filler cap off then you would merely be providing another route for the pressure to be relieved.
Very little oil would exit through the missing filler cap in straight and level flight unless the hole is located in a low pressure area which could cause the oil to be sucked out. Usually the filler cap is located in an area of slightly higher than ambient pressure due to the air being stuffed into the front of the cowling to cool the engine down, so oil being sucked out wouldn't be a problem. However, the usual warning of "don't try this at home children" applies
PS, as FD says, the chances of hearing a dipstick rattling over the general cacophony of an aero engine are non-existant.