I've wondered why he was called "chief" O'Brien even in TNG. What was he chief of, since Geordi Laforge was engineering chief?
On the Enterprise, he was the Transporter Chief.
Well only from about S2-S6.
But I think these are also true about the actor
He is the only actor to appear in both the pilot and finale of two ST shows (TNG & DSN)
He has made more apperances in ST than any other actor except Michael Dorn. Not bad for what was a bit part in the pilot.