Yes rolled oats are absolutely fine.
The kind to be avoided are the unclipped whole oats, which can be sharp and potentially damage the pouch lining.
Clipped oats, or rolled/porridge oats are fine. If you're giving porridge you can also use fine ground/powdered oat such as Readibrek, but if you have a messy ham make sure you feed it where it's easy to clean up, as the powered stuff makes for very gloopy, sticky porridge!
You can use warm water as already said, or you can use kitten milk. Cows milk contains a lot of lactose and this can cause tummy upset if fed in excess, as most mammals become lactose intolerant once past the weaning stage. Some hammies are fine with it and don't experience upset, but to stay cautious and avoid any potential issues, you might as well use whats preferable. Kitten milk has reduced lactose, which makes it more digestible.
You needn't feed any more than a teaspoon of porridge in a sitting, but some hammies won't eat that much. If they leave any, remove it straight after they finish so it doesn't spoil.