Pretty sure there's two types of oats in Vietnam. If you're prepping the oat on the spot and eat it instantly, you need instant oat, no cooking required.

But if you're making overnight oats, you can do with both roll oats or instant oats as you can leave it soak and absorb with whatever you're mixing with it. Preferably use roll oats so the oat doesn't breakdown too much.

https://share.google/aimode/Qmwke2wEmwASePTJx

Here, a quick reference from Gemini.