Later on Odo reveals that when he falls asleep he reverts back to his natural liquid state. Being unconscious is for all essential purposes the same as being asleep, yet Odo retains his sold humanoid shape when he gets knocked out.
When Odo is knocked unconscious by the cave-in while inside the Vortex, he remains in a humanoid form. Evidently, his ability to retain shape doesn't require a constant conscious effort (in fact, it would be too hard for him if it did). Just because someone is unconscious, doesn't mean all their body processes stop working, otherwise losing consciousness would be fatal.
In the beginning sequence when Rom prepares the drinks that Quark requested, he places 4 glasses on a tray and then turns around to grab a bottle off the shelf. When he places the bottle on the tray, there are 5 glasses.
There are 5 glasses because Odo has shapeshifted and turned into an extra glass. We see this when the glasses are thrown to the floor in the holosuite, he 'breaks' and re-forms into Odo. Also, Quark makes the comment "Five glasses for four people?" to Rom, who we already seen is fairly oblivious in season one, and doesn't notice the extra glass.
Why would a blow to the head knock Odo unconscious? It's not like he keeps a brain in there.