You need to check admission requirements for immigration. It it says "proof of onward travel" you may have an issue. You may actually be questioned at YYZ when checking in. When I have bought (2) one way tickets for international travel, the airline has asked me to show them my return ticket-especially if on a different airline-and noted the record etc. They get in trouble for transporting people that don't meet admission regs.

Its unclear from your post if you will have the return ticket purchased when you leave from YYZ. If you do, you won't have any issues. Just be prepared to show it.