Are you coming back to the city or leaving afterwards? If leaving, I'd concur to go with American Taxi. Call the night before and they'll schedule it and have the cab waiting at your time. Website says $34 City to Northlake and then $24 Northlake to O'Hare. That'll likely end up cheaper then renting and parking in the city. If you're heading back into the city, then look into just renting from somewhere close to your hotel.