"UO syndrome" stands for"Ultima Online syndrome", a term coined by Micah in the context of area building on Avlis.
It refers to Ultima Online MMORPG as an example of a hotch-potch assembly of elements, put together with no reason nor rhyme.
"Trying to preserve nicer areas of Avlis, to prevent UO syndrome." --Micah
"UO syndrome" was first used in association with the town of Zvidureth.