Rice cereal is often the first non BM or formula children get, it's thicker and starts the process of them using their tongues to get the food to the back of their throat to swallow it. Some moms (myself included) dislike rice ceral because it's empty calories, it just fills them up but they don't glean any nutrition from it. You can feed it to them by bottle or spoon
I started giving DD oatmeal at 4 months. It's not like instant oatmeal you'd make for yourself. The dry texture is closer to the fake mashed potatoes in a box, thin flakes. You add breastmilk or formula to it and feed it by spoon. I also add fruit to DD's oatmeal and she eats it for breakfast everyday.