You can have two one-ways, LAS-ORD-YYZ and YYZ-ORD-LAS, both times you can stopover in ORD.

Technically, you can probably even do YYZ-ORD-LAS-LAX, with stopover in ORD and a future one-way LAS-LAX (BA's rules require stopover to be en-route, but YYZ-ORD-LAX-LAX seems en-route to me).

Remember that you can no longer change any flights after the first flight on the reservation departs, so for maximum flexibility, do two one-ways and remember that as soon as your YYZ-ORD takes off, your ORD-LAS flight will become fixed.