In a medium bowl, mash the banana with a fork. Whisk in the egg, sugars, vanilla, and baking soda (if using). Stir in the oats and chocolate chips until thoroughly combined.
Transfer the mixture to a well greased, microwave or oven-safe ramekin or bowl, and smooth the top with the back of a spoon or spatula, dotting a few more chocolate chips over the top. (I have used 8- and 16-ounce dishes as well as mini loaf pans; note cooking time variation below.)
Bake in an oven preheated to 350 degrees F for approximately 15 minutes for a 2-cup ramekin or mini loaf pan, or 18 minutes for a 1-cup ramekin. The center should be just cooked through.
Conversely, microwave for approximately 1 minute and 45 seconds, or until the top of the oatmeal is slightly firm to the touch. (If microwaved too long, the oatmeal will become tough. Check on the early side until you know how long this takes in your microwave, extending the time if your microwave tends to cook more slowly.)
See comment section above for serving and storing suggestions. Other delicious options include stirring a few chopped nuts into the batter or spreading the cooked oatmeal with peanut butter and a few slices of banana. The oatmeals can be popped out of the baking dish and enjoyed like a muffin or topped like traditional, stovetop oatmeal. Feel free to create your own version of this speedy, whole-grain, protein-rich breakfast!