Old Age Security (OAS) is the basic building block of Canada’s retirement income system. Canadians build on that foundation, saving for their retirement with benefits from the Canada or Quebec Pension Plan, a workplace pension if they’re lucky enough to have one, and private savings. This short technical paper explains how OAS works, and also sheds some light on the negative impact of increasing the OAS age of eligibility, the program’s sustainability—the number of OAS beneficiaries is expected to almost double over the next 20 years—and several other important considerations for both seniors and younger workers.