Sorry , but no observation deck here at ORD
... Only , when you're waiting for your flight and you got a time you can walk between gate entries and take some shots.
But you can go to rwy28 spot (Lawrence/Mannheim) and take a good shots of arriving a/c's (now we go a W/NW wind so the pattern are good for spotters). In Chicagoland now spotters are threated good ( police or security not asking what are you doing - last time when they ask me was two years ago) so you can come here and take some shots. It is obvious when you will be to close to airport fence or try to take a shot thru the fence hole so you will be threated as a suspect so avoid this all the time.
Wheater now is pretty bad (snowfall and some cold) but I hope weekend will be ok.
The only observation deck here is at PWK
near rwy 34 at southside of airfield.
Regards , Gabriel
Canon 50D gripped + Canon 70D + 17-40 L + 24-105 L IS + 50 f/1.8 STM + 100-400L IS USM + Minolta X300 w/ 35-70 f/3.5