If you're hungry soon after eating your breakfast (or other meals) it's either one or a combination of the following problems:
1. You're not consuming enough calories. If you only eat one serving of oatmeal, that's not going to give you enough energy to do anything in the morning. You'll easily burn off those calories by getting ready for work/school and getting yourself there.
2. You're not adding protein to your breakfast (ie nuts, a protein based side, enough yogurt/milk in the oats, etc). I try for around 40g of protein in the morning. This way I know I won't be hungry until lunch. Some studies have shown that having a larger breakfast and a higher intake of protein causes less blood sugar spikes throughout the day.
3. You're adding too much sugar. (white sugar, brown sugar, agave nectar, honey, rice syrup, maple syrup, etc.) This is pretty self explanatory. You're spiking your blood sugar with too much of these things, causing a sugar crash.
4. You're not hydrating enough. I still don't know why this is a thing, but I guess it is.
5. You're not getting proper sleep. Not enough or poor quality of sleep can lead to increased hunger, especially an increased craving for simple carbohydrates and junk food.
6. You have a medical problem such as diabetes, over production of acid, a thyroid condition, etc.
If you're literally hungry RIGHT AFTER you finish eating anything, it's probably one or a combination of these things:
1. You have leptin resistance. Leptin is a hormone produced in fat cells that signals us to either eat or stop eating. Leptin resistance is common but no exclusive to obesity, but being obese greatly increases your risk for leptin resistance. In addition, some findings have concluded that high fructose corn syrup, triglycerides, and BPA might have a direct effect on leptin and leptin resistance.
2. You are simply eating your food too fast. It takes about 20 minutes for the signals from your stomach wall expanding and the levels of ghrelin dropping to form entirely and relay the message to the brain that it's time to stop eating. If you eat your food quickly, your body does not have the proper time it needs to send these signals. Additionally if you're a distracted eater, (watching TV, reading, etc) you're not giving your brain actual time to process mental satiety.
3. It's entirely in your head. If you're displeased with the food you ate, which it sounds like you were and oatmeal just isn't for you, then your body can force you into eating something it does like, because we're creatures of comfort and habit. In moments like this, you should drink some water or tea and have a bit of walk and reflect on the fact that you did just eat and you shouldn't be hungry. If you still feel hungry after an hour, eat.
4. You have a medical condition such as diabetes, a thyroid condition, a tapeworm or other parasite, or a side effect from a medication to name a few.
If anyone feels hungry all the time, this is a problem and should be addressed by a doctor.
If anyone feels hungry directly after they eat, this really should be addressed by a doctor, especially if it's a large meal and/or they are losing weight.
Also, overnight oats have a different texture than cooked oatmeal, but everyone is offering up the suggestion because it's such a healthy, filling food when it's done right and adjusted to each person's individual needs. When I first started making it, I did not add enough protein and was also hungry after eating it, albeit that was an hour later and not right after. Almost always the culprit when you feel hungry an hour or so after eating is a lack of protein and fiber. Add more to your diet and you'll feel fuller longer.