Jake Sisko often had oatmeal and orange juice for breakfast. On occation he would find it lumpy (DS9: "The Nagus") By 2372, he had began to experiment with different meals for breakfast, including raktajino and makapa bread. (DS9: "For the Cause")
Nog once played a joke on Jake by filling Odo's bucket with oatmeal from the replicator in the security office and dumping it on Jake, leading him to believe that it was Odo he spilled. (DS9: "The Storyteller")
Dried oatmeal was used by Dan Curry as part of the visual effects to create the illusion of the surface of the sun that the IKS Hegh'ta flew into in "Redemption II" (TNG). According to Curry, in the TNG Season 5 DVD special features, oatmeal was sprinkled onto a light table and an electric motor was placed on it to make it vibrate to give it a "sizzle". The flat plane of "sizzling" oatmeal was then wrapped in an electronic device over an imaginary, or virtual sphere. After making a negative of the image and tinting it orange, the final product was given look of the cells seen on the surface of the sun.