Keeps 4 days refrigerated and sets into a solid block — stir in a splash of milk when reheating and it comes right back. Also good cold as overnight oats. Freezes 3 months in portions.
Oats themselves are, but almost all are processed alongside wheat and are cross-contaminated. If you need gluten-free, buy oats specifically labelled as such.
Steel-cut have the best texture and take longest, rolled are the practical middle, instant are pre-cooked and go to mush. Fine when speed is everything.
Over-stirring or too little liquid. Stir occasionally rather than constantly, and add more milk than looks right — it thickens a lot as it stands.