You have to change the grease by pulling out the axle and cones and then clean out the races. The oil hole is to dribble a little thin oil in to re-activate drying out grease. It was useful in the 40's and 50's when racers were on very dusty rough roads but they are more of a talking point today. They also scratch your nice alloy shells when you slide them over.
Words for the "Old" ones at the bike shop decades ago was that the oil port was for adding thin oil to soften up the grease from inactivity over the winter! Even the youngest Campagnolo Record hubs are now over 20 years old. Time to repack.
As a Junior racer long ago rode oil on my track race wheels... loved the sound of the balls dropping at very slow speeds. And no dust covers either, Sprint to Victory.