OAS is based on the number of years you live in Canada. You do not 'contribute' to it, it is a benefit all Canadians receive and it is paid by the Federal government out of general revenues.  There are also clawback limits that begin to 'tax back' the benefit at around $75K income and it is completely taxed back around $120K income. 

So if they have lived in Canada for 40 years and continue to live in Canada, they will each qualify for full OAS depending on their individual incomes in retirement. 

https://en.m.wikipedia.org/wiki/Old_Age_Security