Busy city to busy city domestic can be very tight, ATL-ORD. You have to try an ‘off time’ & date. That may be 1st flight on Sunday morning, or early New Year’s Day.
Domestic loads can swing in a whim, 20 seats to oversold in a heartbeat. International doesn’t swing as fast, protocols for travel slow last minute changes.
It’s an adventure, maybe buy one discounted ticket to get started. I always say have alternative plans. I had a relative at ORD trying to get to SFO then New Zealand, ended up in Ireland, couldn’t get on to the West Coast.
Bad weather also helps standby, depending. Trying to go west out of ORD with bad weather east will lead to misconnects.