Upvote:13
Just flew that route a few weeks ago. No, you don't, you will go through US customs pre-clearance at YYZ (Toronto) and your bags will be checked through. Traveling in the opposite direction would be different.
By the way, be sure to leave enough time at YYZ, it's been horrible recently, I left 2.5 hours for an 08:45 flight and it was not enough time to be comfortable at all. Most of the bottleneck is in security rather than US customs.
I wouldn't worry too much about O'Hare, you have a domestic connection, there is a fast train between terminals if you have to change, and it seems to me that flights out of O'Hare are late more often than not (or maybe just my luck).
Upvote:13
Normally, when entering the US (anyone making a connection still needs to enter the US), you need to collect the baggage, go through customs and re-check the baggage on the connections belt.
However, you will enter the US at Toronto (rather than Chicago), and thus already be customs-cleared on arrival in Chicago, whereby you'll end up directly in the departures lounge. As such, since it's a single ticket, the baggage will be checked through.
This also means that if your flight to Chicago should be very late and you end up missing the connection, your onward trip is the responsibility of the airline. However, unless it is late, you will make the connection for sure.