Yes,
In theory, even my .22 Hornet would also be legal for deer, but I'd only use it for that purpose if my life depended on it.
I'd have to take a closer look at the ON regs, but I think that you might even be required to be carrying a license for deer, bear or moose if hunting with a centrefire in an area during a rifle season for those animals. So in theory, if I'm hunting small game with my Savage 24 with Hornet barrel (also came in .223) and 12 ga lower barrel during the five day moose season in mid Oct, I could theoretically be charged with poaching if I'm carrying Hornet cartriges and haven't a moose tags.
I've heard of COs in my neck of the woods years ago charging a couple of hunters who suspiciously claimed to be hunting rabbits in the woods sometime during the winter with Mini-14s. Coyotes weren't yet common back then, and they were probably poaching deer.
So in addition with being able to hunt deer with .223, do check to see what other big game seasons exist in the area and read up on or ask for clarification about hunting with it during those seasons.
Regards,
Frank