Can I leave O'Hare airport during my layover?

Most US airports have no concept of "transit" and no "airside" where you can make your connection. You land, clear US customs, and then can go wherever you like, whether that's just over to another terminal to catch your plane, or out into the city to explore. So if you have the paperwork to enable the connection, then you almost certainly have the paperwork to go visit Chicago.

There are 4 terminals. United uses mostly Terminal 1 and a little Terminal 2. When figuring out how much time you have, you'll need to include not just clearing US immigration when you land, but clearing security when you go back airside, and possibly some time for moving between terminals. As well, flights into Chicago are often delayed. I think that means in the end you have an awkward layover that doesn't really give you time to leave the building, but is really boring to spend in the airport.

That said, if you're the adventurous type who would love to grab a train or a taxi to a single high-profile attraction, take a look at the tourist pages and see if the timing can work out. You might get two hours somewhere if you try hard.

I did it a month ago, but I had 9 hours between my flights. And I was lucky because I decided to return to the airport earlier than necessary: the CTA derailled and we had to take a bus to return to the O'Hare.

You'll spend almost one hour to go downtown, and another hour to return, so you'll have some 3 free hours. I went to the the Chicago Beans (not the real name!) and liked it very much.

Getting through immigration usually takes at least half an hour and can often take an hour or more. Getting from O'Hare to downtown is another hour; getting back is yet another hour. You need to be back at O'Hare an hour and a half before your outgoing flight. That only leaves an hour to an hour and a half of your layover for actual sight-seeing.