I don't think you are doing anything wrong. I think it is your microwave.
For breakfast I have a bowl of 1/2 oatmeal and 1/2 Rogers Porridge Oats.(oats, oat bran, wheat bran, flaxseed) I make it at work in the microwave in our lunchroom. It never boils over. But on the weekends I had problems with my microwave at home. It is a full size microwave and I am guessing it is more powerful than the one at work. I constantly had the problem of boiled over porridge.
So I bought a smaller, less powerful, microwave and it has eliminated the problem, although I still think the microwave at work is still a bit better in this regard.
Reducing the power on a microwave is changing the on and off time of the magnetron (that produces the microwaves). For an example a setting of 70 could have it on for 7 seconds and then off for 3. Power being on for 70% of the time. The problem is that when it is on for that 70% it is blasting at full power.
If a 70% setting had the power on for 3.5 seconds and then off for 1.5 seconds (5 second cycle time) you would not have the overboiling problem. The reason why they use a longer cycle time is to put less wear and tear on the parts. To make the microwave last longer. Unfortunately the longer cycle times does not suit all things we cook.
Other than buying a different microwave there is a couple of things worth trying. Use a deeper dish. That helped me a little but not enough.
Another option is to put a half filled cup of water in the microwave at the same time. This will absorb some of the power. You may want to bump the power back up to 100% since the cup of water will slow down the cooking time compared to the oats alone. The problem with this is that you are wasting power heating the cup of water. A small price to pay I think.
Since I got the smaller microwave I normally use it when I want to heat something more gently compared to my old microwave. The power output isn't all that different, but just enough to not have messy porridge.