How do you prevent oatmeal from overflowing?
A bigger container is definitely the 'instant' solution.

I've never hear of fruit preventing overflow.  I'd guess the theory there is that it has something to do with the fruit interfering with the bubble to prevent them from forming...but I'd doubt it without A LOT of fruit.

Generally I've done two methods:


Reduce the power and increase the cooking time.  Cut the power by 30% or so and increase the cooking time, this seems to prevent as much of a rapid boil and alleviates the problem.
Determine the time right before it starts to boil over, cut power off, cover it tightly and just let coast the rest of the way.  This is particularly effective if you're cooking in a  vessel with good thermal retention.