If it's a regularly scheduled flight there will be taxis waiting. I believe there are several daily flights arriving in YOW late in the evening, so I don't think it will be a problem.

Avis, Budget, Hurts, and National are all open until 1am. If you are a member of National's Emerald Club, and you're going to arrive after closing, call them. They will make arrangement for you to get the car keys from security, or hide them. Just pick up the keys and go.

Ottawa is not a BIG city, and Kanata and the airport are away from downtown, so traffic won't be too bad. Sometime it backs up and slows down on the freeways, though. At least you'll be going the opposite direction to most of the afternoon traffic.