Overtime pay in Ontario is required by the Employment Standards Act (ESA). Most employees are entitled to "time and a half" if they work more than 44 hours in a single week.
According to this Ontario Government web page
For most employees, whether they work full-time, part-time, are students, temporary help agency assignment employees, or casual workers, overtime begins after they have worked 44 hours in a work week. Their hours after 44 must be paid at the overtime pay rate.
Managers and supervisors do not qualify for overtime if the work they do is managerial or supervisory. Even if they perform other kinds of tasks that are not managerial or supervisory, they are not entitled to get overtime pay if these tasks are performed only on an irregular or exceptional basis.
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A fixed salary compensates an employee for all non-overtime hours up to and including 44 hours a week. After 44 hours, the employee is entitled to overtime pay.
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An employer and an employee cannot agree that the employee will give up their right to overtime pay under the ESA. Agreements such as these are not allowed and would be deemed void. However, an employee can make an agreement to take paid time off in lieu of overtime pay or to average hours of work for overtime pay purposes.
An employer cannot lower an employee’s regular wage to avoid paying time and a half after 44 hours (or another overtime threshold that applies) in a work week. For example, if Josée’s regular pay is $17.00 an hour, her employer cannot drop her regular rate in a week when overtime was worked to $15.00 an hour and then pay her $22.50 (1½ × $15.00) for overtime hours worked instead of $25.50 (1 ½ × $17.00).
There are various industries that are subject to special rules that modify the usual rules for overtime. There are particular kinds of jobs that are exempt from the ESA. These are listed on this page.
Conclusion
The situation described in the question sounds like a violation of the ESA. But it might come under an Averaging Agreement, which is permitted. Under such an agreement weeks with longer hours are averaged with weeks that have shorter hours. There are rules governing such agreements.
Note that the hours listed in the question amount to 35 hours per week. An additional nine hours could be worked in any given week before getting to the 44 hours of work which usually triggers overtime pay.
The linked pages include official contact information for ESA information and enforcement.
