I realize this post is old, but in the event someone looking for the same information find this post: -Street Parking overnight is free on most streets downtown, unless a winter parking ban is in effect. The official rule on that is "There is no parking on city streets between 1 a.m. and 7 a.m. when 7 cm or more is forecast by Environment Canada in the Ottawa area. This includes any forecast for a range of snow of more than 7 cm (for example, 5 to 10 cm)." If snow is forecast, best to check on the city's website to verify (Ottawa.ca). T That being said, if you park overnight on a weeknight, just be aware that there are often no-parking restrictions that kick in as early as 5:30-6:00am, so you may have to get up early and move your car. Shouldn't be an issue on a Friday or Saturday night though. -The underground garage at World Exchange Plaza is open 24/7 and is free on Saturday and Sunday. That being said, they do have "No Overnight Parking" signs at all entrances so YMMV as they have been known to issue tickets to overnighters on rare occasions.