Since maltodextrin doesn't seem to be available anywhere in Iceland, would it be a good idea to supplement dex in my pwo shakes with oatmeal? What would I lose/gain from using oatmeal instead of malto in that mix?

Don't know what you'd gain/lose, try it and see. :) I honestly think that it will be fine, and I actually like the idea of oats better than malto, at least for most bodybuilding purposes. Generally speaking I just favor oatmeal (and/or other foods) over malto (and dex and WMS too), especially when I'm cutting calories.

I take it you're not a fan of insulin spiking then? It seems pretty advanced stuff so I thought I might benefit by trying dex with my whey powder. Last time I was on a bulk I used a premixed whey+dex+malto "weight gainer" shake and it seemed to work fairly well, but I have no experience trying to mix the stuff myself. Been using bananas or oats or sometimes both during my summer cut in my pwo shakes.