Hi I'm Ben,
I wouldn't be able to help you about spotting at O'Hare from outside the airport because I rarely leave O'Hare when I'm in Chicago but I know the airport like my pocket. I always go through O'Hare on each trip when I fly American to and from Brussels.
From Terminal 3, American's terminal, you can have great views of aircrafts taxiing, taking off and landing on RWY 09R/27L and 22L/04R. O'Hare is busy all day long, you will see mostly American and United aircrafts. Also from Terminal 1, United's terminal, you can spot various aircrafts movements on RWY 09R/27L and 32L/14R, the airport's longest runway. Sometimes at the end of the day, if the runway in use for landing is 04R, you may see as many as six aircrafs lined up one behing the other on approach for landing! It's unbeleivable. I always enjoy spending time at O'Hare, and it's a beautiful airport.