I imagine Kira would have command authority on all Federation assets within the Bajoran Sector as part of the agreement between Bajor and the Federation. However if Kira were to go aboard the Enterprise in a different sector for example i'd imagine she would be an observer/consultant and only be given command authority if Picard authorised it.
I did notice though that once Worf arrived on the scene Kira commanded the Defiant a lot less.
I are totally right. On a side note. I've always wondered IF Worf had been injured during the events of "Rules of Engagement"
could O'Brian take taken command?
He says he would have. But could he have? He's only an NCO.