Kerrrist, you don't need to buy a special powder for oat milk. You need a food processor, water and rolled oats.

Grind 1 cup dry oats in the food processor as fine as you can. Put in a bowl/jug/whatever with 1 tsp salt and 3 cups cold water OR to cover and then about 5 cm over the top. Stir. Put in fridge overnight. Stir again and sieve through muslin into a jug, pressing firmly to extract as much juice as possible. Hey presto oat milk. Stir briskly before using.
If the oatmeal swells more than the water you put in, add l-2 more cups of water and stir briskly and leave for an hour to soak. You can get at least 2 soaks out of a cup of ground up rolled oats if you are feeling frugal, but one is better.
The drained oats make quite a nice breakfast cereal with fruit, yoghurt etc. and can be used up in make your own bread. I have used them to crumb fish. They will keep for 1-2 days in the fridge in moist condition. After than toss them or feed them to the dog. Unless you will use them to make uisghe of course, in which case you'll need to add a lot more water - refer @taleya.