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.