After trying the non-starchy-veg-only approach, I've found that starchy vegetables (and fruit) really are the missing link in having a good poo. Have some sweet potato and I guarantee you'll see improvement.
Not sure what "all bran" is, but if you really want some oatmeal, I say go for it. Look up a traditional method of preparation, and as long as you don't have major gluten-intolerance issues, it won't hurt you.
“The whole concept of a macronutrient, like that of a calorie, is determining our language game in such a way that the conversation is not making sense." - Dr. Kurt Harris