I always thought it was silly that they sent O'Brien on the mission, especially in make-up that made him look like Kletus the Slack-Jawed Klingon, when it would have made much more sense to send Jadzia, who already knows a ton about Klingon culture and could have done the simple job of setting up the laser-beam-thingies just as well as O'Brien could.
Out-of-universe, I suppose the reason would be this much-ballyhooed skin condition of Terry Farrell's, making her unable to tolerate too much make-up. But in-universe, I think it was a stupid oversight, especially since Kira was remaining behind to command the station. Maybe they needed someone to stay behind who could command the Defiant just in case, but then Kira's done that before too.
I've heard sunlight and her skin don't mix time and again, but now makeup too? Man at times I'm almost wondering if she doesn't just play that up for convienence's sake. Certainly she pitched a fit when a few Jadzia soundclips weren used in season 7 that that lovely montage in What You Leave Behind awkwardly didn't feature Jadzia. Sorta indicates she's not a team player, quitting or not in season 6.
The Dax character would have made a lot of sense in this situation though, I'm surprised I never thought of that before. I figured the "Kletus the slack-jawed Klingon" was just another variation of "O'brien Must Suffer." I never liked Kira commanding the Defiant at all. Sure allies share their bases and what not, but I've never heard of them turning their warships over to foreign officers.