If there is no lightning, you should be able to do IOA, and still have fun.
In the event of lightning, though, do US. Most attractions are indoors (Shrek, Jimmy Neutron, Twister, Earthquake, MIB, BTTF, ET, Terminator, HMS). You may want to wear a poncho to get from one attraction to another. During a thunderstorm, we usually see a Horror Make-up Show (or two, depending on the severity of the storm). During a storm is also a good time to have your meal (La Bamba is right across the street from the HMS, so we sometimes make a dash for it after the show lets out.)
We never let the rain ruin our trip. Besides, it cools things off a bit!