Great question — answering as the UphillTech team:
You could use a mortar and pestle, if you have a good (and large) one - though it would take a lot of time and grinding to make it work, and probably small batches to fit your mortar and pestle size, it is doable, especially if this is a one-time use. you would probably not want to do this often, though. You might try a blender, it's very similar to a food processor if you happen to have one. I would not recommend an immersion blender, as the oats (and flour) would fly everywhere (as opposed to liquid which holds together a bit more), but a regular blender should work well (also, keep it covered, though). If you have a coffee grinder, that's also a possibility - it is intended to turn beans...
Full answer by Megha: https://cooking.stackexchange.com/a/76778 (CC BY-SA 3.0).
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