The month after your 65th birthday.
The previous government had proposed a gradual shift towards beginning OAS at age 67. This has since been repealed, making 65 the starting age for the foreseeable future.
You can defer receiving your OAS pension for up to 5 years and receive an increase of 0.6% for every month you delay receiving it, up to a maximum of 36% at age 70. Unlike CPP, there is no ability to receive it before age 65.
For the period from October to December 2016, the maximum OAS pension amount is $578.53. OAS benefits are adjusted quarterly (in January, April, July and October) if there are increases in the cost of living as measured by the Consumer Price Index.