http://www.hrsdc.gc.ca/en/isp/pub/oas/oas.shtml
If you fall into either of the categories below, you may be eligible to receive the Old Age Security pension.
Category 1 - People living in Canada
You are 65 or older.
You live in Canada and are a Canadian citizen or a legal resident at the time your pension is approved.
You lived in Canada for at least 10 years after reaching age 18.
Category 2 - People living outside Canada
You are 65 or older.
You left the country and you were a Canadian citizen or a legal resident of Canada when you left.
You lived in Canada for at least 20 years after reaching age 18.