What does an oxidised geuze taste like?

I've, personally, never experienced it, but I've certainly heard of people talking about said character. I think the reason that you don't taste it very often is that if there is Brettanomyces in the beer in question, the dissolved oxygen will be used by it to manufacture acetic acid and ethyl acetate instead of trans-2-nonenal.

 

I would like to add my uninformed opinion to this to say I think that it is the oxidation characteristics itself in sour beers that make the funky flavors enjoyed by many. I would think the earthy flavors in particular would come from some form of oxidation.

In my experience making some really nice sours myself homebrewing, oxidation is the name of he game. I would hot side aerate the mash, aerate transferring, and let the seasonal temperature swings of 80-30s let my lid fermenters breathe over the year. The hops id use would be at least 2 years old in a paper bag left in my attic. Once I started employing such old-tradition techniques I noticed I was getting the flavors and acidity I wanted.

So again- I think oxidation is already very much a part of what you are already tasting.

 

Those flavors and aromas are byproducts of metabolism from Brettanomyces sp. and are likely phenol in nature. Other microbes are certainly involved, but the lion's share of the flavor profile of lambic/gueuze is Brettanomyces related.

If anything, you want to prevent cold side oxygen ingress as much as possible, as Brettanomyces sp. will make acetic acid and ethyl acetate in the presence of oxygen.

It certainly is, as oxygen exposure is inevitable, but it is not important in the way that you posit.