I am currently an infusion cooler MLT brewer, but I wanted to try mashing in a brewpot on the stove and just using the cooler to lauter with. Will pouring the mash into the cooler in order to lauter cause an extreme amount of oxidation that will ruin the beer? If so, why does oxidation before boiling even matter?
You're talking about Hot Side Aeration. The theory is that oxygen bonds readily with the pre-boiled wort and changes the lipids of the wort increasing the rate of spoilage. HSA isn't really a problem for homebrewers because the amount of splashing is minimal and the beer isn't typically kept around long enough to experience the negative effects of HSA
You are going to get mixed responses on this, but I will say no, you won't pick up off flavours by doing this. In theory, it is certainly possible that you will oxygenate your mash, but if you are careful about how you transfer the mash you are going to introduce only a small amount of O2. Homebrew beer is much more robust to oxygenation effects than many commercial beers (especially BMC), so there is much less worry here. Most of the panic about HSA has come from people extrapolating from pro-breweries down to the homebrew scale without thinking about the differences carefully enough IMHO.