Update- here's how the situation played out: UA agent in RDU could only check the bag through to YOW (or so she said), however the agent at the connecting bag drop on YYZ removed the tags, and more importantly from the reservation as well. There were no issues when I explained the situation, but I was advised that the most important thing when doing this is having the note that you're traveling with bags removed from the reservation, or you'll have trouble as another poster described above.
As a pleasant aside, OLCI is now available from RDU.