It it ok to use peanut butter instead of brown sugar in oatmeal?

Yes. Peanut butter instead of brown sugar is a good idea, but it provides protein along with the oats. It also would be a better choice for people concerned about their blood sugar because you would be less likely to have a blood sugar spike with peanut butter and oatmeal than with brown sugar and oatmeal.
Yes BUT sparingly. A 1 ounce serving of peanuts has 166 calories. Peanuts have 14 grams of fat but only 1.9 grams of saturated fat as well as 4, 393 mg of omega 6 fatty acids but only 0.8mg of omega – 3 fatty acids. Peanuts & peanut butter are high in calories.