Yes, hamsters can eat oatmeal in small amounts.

Oatmeal can be a nutritious and safe treat for hamsters when prepared properly. It is a good source of fiber, vitamins, and minerals.
When offering oatmeal to your hamster, make sure it is plain and cooked. Avoid flavored or sweetened oatmeal that contains added sugars or artificial ingredients. Plain, cooked oatmeal without any additives or toppings is the best option for your hamster.
Cook the oatmeal according to the package instructions, using water and not milk. Allow it to cool down completely before serving to your hamster. You can offer a small amount of cooked oatmeal as an occasional treat, alongside their regular diet of hamster pellets and fresh vegetables.
Keep in mind that oatmeal should not replace the main diet of a hamster, which should primarily consist of hamster pellets. Oatmeal should only be given as an occasional treat due to its high carbohydrate content.
Introduce new foods gradually and monitor your hamster’s response. If you notice any digestive issues or other adverse reactions, discontinue feeding oatmeal and consult a veterinarian for further guidance.
Also explore the compatibility of these foods for your hamster:
Watermelon, Tomato, Spinach,
Remember to maintain a balanced diet for your hamster and provide a variety of foods to ensure they receive all the necessary nutrients. If you have any concerns about your hamster’s diet or want to introduce new foods, it’s always best to consult a veterinarian with knowledge of hamster nutrition for guidance.
Further Reading :
https://www.thedodo.com/dodowell/hamster-food