There's some weirdness with who's Sisko XO and when, but on this run of watching DS9, I've been asking myself if Dax got promoted to Lt. Cmdr. after "Call to Arms" to be Sisko's XO during the Defiant's attachment to Starbase 357 (before she becomes skipper of the Defiant, temporarily, at least).
There's certainly a few scenes where Bashir, O'Brian and Nog hand her reports as if she's acting in that role.
Anyone have any confirmation or at least any further supporting evidence if this was the case?
PS I'm largely only talking about the stent between "Call to Arms" until "Favor the Bold"