At the beginning of Way of the Warrior the entire DS9 crew is busy flushing out a Changeling in what turns out to be a drill. But the whole thing ends when Odo grabs Dr Bashir by his shoulder, in a pretty indistinct part of The Promenade. What was the point of this drill? He just grabs Dr Bashir for some reason and the drill is complete.
As the episode opens we see station personnel paired up and sweeping each level with low-level phaser fire in order to find a changeling. As we saw in DS9: The Adversary, this is sufficient to force a shape-shifter to revert but not high enough to incapacitate them (something that's especially important since this is actually a 'hunt-the-evil-changeling-infiltrator drill' with the main part played by our old friend Constable Odo).
Bashir and O'Brien are a two-man team in charge of a small security detachment. Having been told by Sisko that Odo is somewhere in the vicinity they direct their troops to sweep around while they take the main concourse. Bashir foolishly stands next to the panel and instead of waiting to be revealed now that he's surrounded, Odo attacks and 'kills' Bashir. O'Brien then 'shoots' Odo and the drill is over.
Just then, an arm reaches from behind Bashir and grabs him by the shoulder. As a startled Bashir turns around, we hear the sound of a morph.
We see the tail end of a morph as ODO separates himself from the shop directory. Just then, O'BRIEN steps up behind Odo and aims his phaser rifle at the constable.
O'BRIEN: Bang. You're dead.
ODO: And so is Doctor Bashir.
The exercise was to learn to counter changeling infiltration. Just after Odo grabs Bashir, O'Brien trains a phaser rifle on Odo and said "You're dead" meaning he would have shot a different changeling; Odo counters "So is Bashir", meaning he caught Bashir off guard and a different changeling would've killed the doctor in the same situation.