Getting through Customs at O'Hare is fairly painless. Step 1 - no cellphones. If you are caught talking on your cellphone while inside Customs, they will confiscate your phone. Step 2 - Get in the correct line. There are two lines to get through Customs. One is for U.S. Citizens, and one is for everyone else. Have your forms filled out before you get in line. Your airline should have given you the proper form before you arrive. There's extra forms along the back wall as you walk into the Customs area. The line to get through Customs moves fairly quickly. It all depends on how many flights arrive at the same time.

If you are unfortunate to arrive at time when many International flights arrive at the same time, you could well be waiting in line for upto 2 hours (dependent on number of Immigration Officers on duty).  You may wish to consider allowing extra connecting time when making your travel arrangements (if you are an International traveller) when connecting to a Domestic flight; as not only do you need to clear Immigration but also collect your baggage, recheck your baggage and ride the train to the Domestic terminals before clearing security again.

After you cleared Customs, hang on to the form that was stamped. You'll need it later. Then you'll go to the next room to claim your baggage. There are free trolley carts to hold your baggage. After you get your baggage, you'll need to give the stamped form to the Customs agent at the exit.  

If someone is meeting you at the International Terminal, they will be in the hall directly outside the sliding doors pas baggage claim. If you're transferring to a domestic airline to continue your journey, you'll need to make your way to the appropriate terminal to check in for your next flight. There are plenty of signs to guide you along. You will be arriving in Terminal 5. Your connecting airline will be in either Terminal 1,2, or 3. There is a people mover tram that stops in all of the terminals. You can't walk from Terminal 5 to the other terminals. If you are taking a taxi, there's a cab stand at Terminal 5. If you're taking the CTA train, follow the signs to the O'Hare station.

There's a McDonalds after you go through baggage claim if you need an immediate fix of American fast food.

 The airport is subject to frequent closures at certain times of the year due to adverse weather, e.g. lightening hitting the ground.  Airlines are pretty good at getting people booked on to alternative flights when you are grounded, but it could take up to 2 days to get back to normal when aircraft isn't where it should be.   Unless you're on a package holiday, travellers could well find themselves stranded.  Queing at a recheck desk doesn't really help, you get told the same thing as phoning your airline.  Save yourself the stress if this happens- try getting put on standby for anything left that day and having a hotel on reserve, Chicago's a cool place to hang out.