How can I turn rolled oats into flour without a food processor?

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 into powder, this is just a different grain. You might want to make sure it's quite clean unless you want coffee flavored cookies, though.

And final option, if you have a spice grinder, that should also work. Some people get a spare coffeee grinder for the job, or if you have a little hand crank grinder (like some people use for pepper or cinnamon or such), that would also work - though again, lots of work and small batch sizes, but possibly worth it for one time.

Additionally, if you toast the oats a bit, they should be dryer and easier to grind (and toasting gives a bit of flavor) - this will help especially if you're using one of the hand methods, like mortar and pestle or tiny spice grinder.

We often use a coffee grinder to make almond flour for my son who is on a very restricted diet. We use a simple 19.99 blade grinder rather than a burr grinder. We've also used it to create powdered sugar from Xylitol and from ordinary cane sugar, and tapioca starch from tapioca pearls.

Good luck!

I just buy oats from my local supermarket and then I turn it into a flour for making my morning porridge in the microwave. The result is a creamy delicious porridge with bananas or tinned fruits, that I enjoy all the year around and never get fed-up with it. I found the best way to turn oats into flour, is by using a blender on fast speed or as I do, by using a 400 Watts Braun hand blender, this too on fast speed. When using the microwave to make your porridge, blend the four into some COLD milk, then stir in hot water, but not too much, otherwise you will end with a watery porridge and not a thick creamy porridge. It was my idea to use the oats as flour, so that once in the stomach, it will easily attach itself to the cholesterol and then you know the rest. It's a great idea if you want to reduce your cholesterol and lose weight.

Use a sift. It may take long, but the result is fine powder. Take some rolled oats and place it on the sift. Put the sift on top of a bowl or container. Rub the oat against the sift. After a while, you should have oat flour.

An ordinary flour sift works. You just have to be patient. It takes a long time and you can't press to hard. Perform a sort of stir, rub motion.