ORD is a huge airfield with seven runways spread out so far apart that it is impossible to watch them all at once. Traffic is largely dominated by United and American, and for the most part, aircraft are of the smaller and/or commuter variety. There are plenty of heavies and internationals, but they don't make up a tremendous percentage of the endless traffic here.
-ORD Hilton. Located right in the heart of the airport, the upper floors of the hotel will give you a great view. Of course, because O'Hare is so spread out you'll only be able to see half of the airport from either side of the hotel, so best to get a room facing the south side and head to the parking garage to see the north side.
-Terminal 2. Various points along Terminal 2 offer a view of 9R/27L, which is often the most widely used runway, especially by the big birds. From nearly every gate towards the end of this 2 concourse, "Y" shaped terminal you can get some sort of view of the action.
-Terminal 1, Concourse C, Gates C-1 and C-31. C-1 gives you a decent view of the south end of the field, although Concourse E obstructs much of the view. Gate C-31 is at the opposite end of the concourse and gives you a nice view of the north side of the field. Most of the heavy action, though, tends to happen on the south side.
-Central Parking Garage. Located right behind the Hilton in the middle of the airport, the parking garage gives an awesome view of the airfield from the upper floors. The south side of the field is greatly obscured by the Hilton, but the north side is completely unobstructed, and runway 9L is right in front of you, offering great shots of takeoffs if that runway is being used.
-Manheim Rd. Alongside the eastern edge of the airport lies Manheim Rd., and from various places along the road you can get a clear view of the action. Approaches on 27L are great from here. There are several places in the area which offer greater protection from traffic and security, such as the parking garage of a nearby convention center and a wide-open vacant lot just east of Manheim on North River Rd. Approaches on 27L fly right over your head.
-Devon Ave. On the north side is Devon Ave. Along this street there are several office bulidings with parking garages. From these garages you'll get a great view of approaches on 22R.
PRE-9/11 SECURITY REPORT:
-Security is very tight in the central parking garage. It is regularly patrolled by security on foot, on bikes, and in cars, often bringing dogs with them. The garage is so big, though, that you should be able to keep away from them. Even when they do track you down, they don't always kick you out. I was approached by a security guard once he saw me pull out my camera, and all he did was say, "Be careful, they might take your camera away." Certainly security at ORD is strong, but at least it's tolerant and forgiving.