Since no one answered, I'll ask again:
Since when is O'brien chief of engineering? I've never heard of him having that title. He was Chief of Ops and Transporter Chief. I haven't seen 100% of ds9, so if I'm wrong, and he holds that title for a short time, please correct me.
He isn't. But DS9 doesn't have an engineering chief best I can tell, and O'Brien's job on the station seems the most equivalent to a starship's engineering chief out of all the main characters on the show. That's why I put him in the poll.
Whether he would ever be chief of engineering on a starship I don't know. Is there a requirement that the head of engineering must always be a commissioned officer? Can an NCO hold the position under any circumstance? How is it in the USN? (Most ST rules and guidelines in this area are modeled on USN ones)
Who is over engineering on the Defiant?