I use a lot of oats, and not just in beer.
The flaked oats you get from the LHBS are bought in 50# bulk sacks and are as plain as they get. The flakes are very soft.
The 2lb 10oz round containers with Old Fashioned oats sold in supermarkets (Quaker, varies store brands) can be a bit different from the LHBS bulk sack ones, also depending on brand. All those are a bit crispier and taste toastier, so I reckon they've received a slight toast for flavor improvement. Among those, some brands (e.g., Quaker, Shurfine) seem slightly toastier than others (e.g., Aldi's), but this could be illusion or sample/lot differences, I haven't put them side by side for a taste test.
Any of these can be used, as well as instant oats (without additives). It helps speedier conversion by milling them and if used in larger quantities a cereal mash may add a bit more efficiency and easier lautering later.
If you want more character a slight toast in the oven will give you that. If you get them really dark they need to outgas for a week or longer, as with all grains.
Simpsons sells a crystal oat, Golden Naked Oats, which are like a C20, but from oats. Quite a unique product with an awesome flavor and "texture."