Also, your parents may have requested to have some income tax withheld at source from their OAS payments. They can check their T4 OAS slips to see if any tax was withheld.
Full pension January - March 2020 is $613.53/per person age 65 based on 40 residence years in Canada. If less than 40 years, do the math, divide $613.53/40 x # years resident in Canada.
the other thing to consider is having tax deducted or not from the OAS the pensioner receives.
You may qualify for a full OAS pension in one of two ways:
You resided in Canada for at least 40 years after turning 18; or
You were born on or before July 1, 1952, and
on July 1, 1977, you resided in Canada, or
after turning 18, you resided in Canada for a period of time prior to, but not on July 1, 1977, or
on July 1, 1977, you possessed a valid Canadian immigration visa.