When selecting a tree, the plant hardiness zone can help you determine if it is likely to survive the weather in your area based on the average climatic conditions of years past.
Ottawa is considered to be in zone 5a. So while you may have success growing trees which are rated for less hardy zones (eg. zones 5b, 6, or 7) your tree will have a better chance at survival if it is hardy to zone 5a or a slightly cooler zone like 4a or 4b.
Note that if a tree is said to be “hardy to zone 4” then it can likely tolerate a minimum temperature of −34.4 °C. A reference table relating growing zones to temperature can be found here.