The Old Age Security (OAS) pension is a monthly payment available to most people 65 years of age and older who meet the Canadian legal status and residence requirements. Your employment history is not a factor in determining eligibility: you can receive the OAS pension even if you have never worked or are still working. More details on eligibily.
The OAS recommends that you apply 6 months before you wish to begin receiving your OAS pension.
If your income is greater than $69,562 the government will begin reducing your OAS. At or above the income of $114,815 there is no OAS payment. You can find the formula to calculate the reduction in your OAS here.
You will receive a higher OAS pension if you delay your OAS after 65 years old. The increase is 0.6% per month or 7.2% per year. The DTU has calculated that if you begin receiving your OAS at 66, at 79 years old you will be even, in the sense that the total amount you received between 66 and 79 is the same as the total amount you would have received between 65 and 79 if you had begun receiving your OAS at 65. After that you will be ahead. At 67 you will have to wait until 81, at 68 to 83, at 69 to 85 and at 70 you will have to wait until you are 87 to begin gaining. Which year you will choose to begin collecting will depend on your life expectancy and your expected need for income over your life.