I have used oat straw and like it. Pesticides are a concern with any type of straw, so you just have to find your comfort zone there. Having access to the farmer and understanding their methods can help.
Another thing you should know is that Oat straw often still has seeds in it. I have had oats pop up all over our sheet mulched beds. They look like grass. It wasn't a big problem for me, just a little extra weeding.