It really is as simple as blending rolled oats and water in a 1:4-ish ratio for 30 seconds or so, and straining the result twice. Adding sugar is optional. It stores pretty well in the fridge, maybe up to 5 days. Trust your nose!

Personally I don't make it very often, as my main use for milk is in cappuccino, and plain oat milk doesn't steam very well. The barista editions you can buy have some added extras (fat, sugar, proteins, stabilisers) to improve the characteristics for steaming.

I totally understand the convenience factor of store-bought too. If you don't have a blender on standby it's a bit of a hassle. And the store bought stuff is shelf-stable for weeks when sealed.