ORD crew are very good with customer service, and unfortunately that also includes weighting everything it seems. Especially on full flights, they really wanted to make sure they have a good estimate on the amount of weight on the plane. The few times I flew out of ORD I've seem agents weight everything, and if you are overweight they will ask you to check-it (carry-on) or reduce weight (for checked luggage).

Luckily the few times I flew out of ORD I'm within the limit, but I don't know will they be flex with F-counter check-ins as JFK seems to be (once checked in 32kg )