Odo gets knocked out a couple of time, firstly (As @KyleJones has pointed out) in [DS9: Vortex][1] by what appears to be a blow to the shoulder. At this point he's in changeling form.

> But as they pass through the tunnel, there's a direct hit and rocks
> come sliding down on top of Odo...
> 
> **CRODEN :** *Odo!*
> 
> He leans down and examines Odo, **who is unconscious...**

![enter image description here][2]

It's not especially clear what caused his injury but from reading the script, it seems likely that the writer originally intended there to be a lot more rocks!

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He also gets punched out (by O'Brien) in DS9 : The Assignment but this is whilst in human (non-changeling) form


  [1]: http://www.st-minutiae.com/academy/literature329/412.txt
  [2]: https://i.sstatic.net/f5dGh.gif