I don't want to create too much panic. I've never had anything unrecoverable happen through Chicago (my luggage has always shown up the same day and I've always gotten to where I've been going the same day, but I've been rebooked alot (even the day when my MCO-ORD-YEG flight became MCO-ORD-DFW-YYC-YEG, that was Canadian Airlines-American Airlines way back when). My worst experiences have been in winter, so maybe May will be good.
What time of day are you flying out? The early morning (7:30?) or the mid-day (12:45?)? The earlier the better (I hope you're 7:30). The reason is that once you get to Chicago, United usually has 2 more direct flights after the one you will be on. If anything happens to you or your luggage, that's two more opportunities to get to MCO.
The best advice I can give you that we do every time we fly, but especially through problem hubs is to have both the number for United reservations and a list of alternate flights out of Chicago. If your flight is cancelled or unreasonably delayed, call the number and try to get rebooked. The reservation number can help you just as easily as an agent at the airport (the one that every other passenger will be mobbing when a flight is cancelled).
Other than that I would say that always remember that you are going to the most magical place on earth and you WILL get there.