While the terms oatmeal and porridge can be used interchangeably to describe a warm dish made by crushing or chopping oats and combining them with milk or water, the term oatmeal refers to dishes made entirely from oats. Some types of porridge contain other ingredients, such as rice.
Porridge can be prepared with oats, barley, buckwheat, corn, wheat and certain types of legumes. When it is prepared with oats, it's called oatmeal or porridge. If there are no oats in the dish, it's plain porridge. A dish called gruel is also similar to porridge, but it doesn't contain any milk and is much thinner in consistency.