If your net world income exceeds the threshold amount ($79,054 for 2020), you have to repay part or your entire OAS pension. Part or your entire OAS pension is reduced as a monthly recovery tax.
You must pay the recovery tax if:
your annual net world income is more than $79,054 (for 2020, in Canadian dollars), and
you live in a country where the non-resident tax on Canadian pensions is 25% or more