Since you didn't mention how you cook your oatmeal, I'll use the ways I've cooked mine as a reference for how to answer your question.
I usually cook old fashioned oats (lacking those I use quick oats) with brown sugar in milk, on the stovetop (but I used to do it in the microwave). I've done it in water before, but not enough to where I want to say much about it. I add a fair amount of milk, but I cook it until most of it's absorbed on the stovetop (I made it soupier when I cooked in the microwave).
In my observations, oatmeal will overflow if you cook it on high heat for too long.
For the stovetop, if you use a lower heat and cook it longer, or turn it down before too long, you shouldn't have particular problems (and you'll be less likely to burn your milk that way on the stovetop). On our stovetop, on one of the two larger burners (in a glass or stainless steel pot), it doesn't overflow if the heat is set on 4 (with the options being, from lowest to highest, these: low, 1, 2, 3, 4, 5, 6, 7, 8, 9, high; 5 is medium heat).
For microwave cooking, there's a sweet spot you just have to learn to find, and I don't remember what point that was. Stop cooking it before it overflows, and expect it to overflow if you cook it too long. (I didn't deal with temperature settings or anything; so, there are other microwave solutions, I'm sure).
FYI, I'm not recommending using a microwave for oatmeal (although it can taste pretty great). I personally think it's healthier to cook oats and other grains longer than the microwave allows palatably.
I have found that some substances can change the consistency of things like milk porridge. I'm not sure that fruit prevents boiling over, but it's possible. I know food grade diatomaceous earth seems to reduce curdling (like if you boil some jam in your oatmeal, the jam might cause it to curdle into acid cheese), anyway.