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 a certain amount, the government will begin reducing your OAS. 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.