Upvote:3
As of March 2016, you can't store your luggage at O'Hare:
Can I store luggage at the airport?
Currently there are no facilities or provision for the storage of luggage at O'Hare International Airport.
Your best bet (as noted by @MichaelHampton in the comments) is probably to give it to the airline for your ORD-NYC flight, and then leave the airport to do your sight-seeing.
You didn't ask but I'm telling you anyways: Your best bet is to take the CTA Blue Line to & from downtown. Allow 1 hour each way. Assuming that you want to get back to the airport by about 20:00, and that you won't actually be free of the airport until 14:30 (allowing for travel delays and the time to claim & re-check your bags), you still should have a good 3.5 hours in the downtown area. If it's a nice day, and you have some money to spare, I highly recommend a boat tour from the Chicago Architecture Foundation.