with oats/oatmeal

Maybe 'hot pack,' though 'heat pack' is understandable.

Does that mean you put dry oats in a bag and heat them, or use a baggie of hot cooked oatmeal, or you spread cooked oatmeal on your skin and put a hot pack on top?

'Is supposed to' is okay, or 'People say it works wonders,' 'I've heard it's really great,' etc.

Thanks :)

Yes indeed, you put dry oats in a bag and then you microwave the bag. It's great :)