Oatmeal is nutritionally identical to cereal for the obvious reason that that's what it is.
Here's the nutrition sidebar for Quaker Oats rolled oats versus Cheerios:
+---------+----------+
| Oatmeal | Cheerios |
+--------------+---------+----------+
| Serving size | 40g | 39g |
+--------------+---------+----------+
| Calories | 150 | 140 |
+--------------+---------+----------+
| Fat | 3g | 2.5g |
+--------------+---------+----------+
| Carbohydrate | 27g | 29g |
+--------------+---------+----------+
| (sugar) | 1g | 2g |
+--------------+---------+----------+
| (fiber) | 4g | 4g |
+--------------+---------+----------+
| Protein | 5g | 5g |
+--------------+---------+----------+