Non-resident tax on pension and OAS income is based on tax treaties that Canada has with the county where you are living. You might be pleased to learn that Canadian pensioners living in the US and receiving OAS and CPP are tax excempt. Refer to the information at the following link: https://www.canada.ca/en/services/benefits/publicpensions/cpp/cpp-international/before-apply.html 

Review the section titled Non-Resident Tax Rates by Country