The current rule
Section 38(3) of By-law 2023-074: "Heat shall be provided from September 1 to June 15 of every year, and shall be provided and maintained so that the room temperature at 1.5 metres above floor level and one (1) metre from exterior walls in all habitable rooms, is at least 20 degrees Celsius."
The measurement point is part of the standard, not a detail. A reading taken beside a window or at floor level is not the test the bylaw sets, and a dispute about compliance is often really a dispute about where the thermometer was.
That season is long — nine and a half months — and materially longer than Toronto's October 1 to May 15, though the temperature is a degree lower.
The exception that does most of the work
Section 38(4) provides that subsection 38(3) does not apply "to a rental unit in which the tenant can regulate the temperature and a minimum temperature of 20 degrees Celsius can be maintained by the primary source of heat".
Both limbs are required: the tenant must actually control the thermostat, and the system must be capable of reaching 20 degrees. A unit with a thermostat wired to a boiler that cannot get there is not exempt.
Section 38(5) adds that a rental unit "shall not be equipped with a portable heating appliance as a primary source of heat", which closes the obvious workaround for a failing system.
The trap: two bylaws, one repealed
By-law 2017-007 was Oakville's previous property standards bylaw and it set 21 degrees. It has been repealed and replaced by 2023-074, but copies remain reachable online, and the Town's own summary page describes the heat obligation without stating the dates.
If you are checking a number for Oakville, confirm which bylaw you are reading before relying on it. Both documents look official because both are official — one is simply no longer in force.
This is the same failure mode as the widely-repeated stale Toronto dates, and it is why every figure on these pages names its bylaw and its section.