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.
I raised the same point a while back, he is an engineer and seems to serve as chief engineer of the Defiant but is technically DS9's operations officer.