I live in Ontario and use a .308 for deer
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Only part of Ontario has a calibre restriction. The southern part (generally). Here is the required info
The holder of a small game licence may not use a rifle of
greater calibre than .275, except a flintlock or percussion
cap muzzle-loading gun, for hunting small game in
the counties of Brant, Elgin, Essex, Huron, Lambton,
Middlesex, Northumberland, Oxford, Perth and
Wellington and the regional municipalities of Chatham/
Kent, Durham, the former regional municipalities of
Haldimand-Norfolk and Hamilton-Wentworth, Halton,
Niagara, Peel, Waterloo and York and the City of Toronto.
Concerning deer hunting, some of the southern units do not have a rifle season.
http://www.mnr.gov.on.ca/stdprodconsume/groups/lr/@mnr/@fw/documents/document/239849.pdf
pg 7 of that link has the table with the required info. Without looking at a map, I'd hazard a guess that if you lived south of a line from barrie to ottawa you will not be in a WMU with a rifle season. Now if you can visualize Ontario, you will see that is a relatively small portion of the province. I live in Muskoka and can use whichever size firearm I want.