If you are under CSRS-Offset, the “offset” is a reduction in your CSRS benefit in the amount of the Social Security benefit you earned while covered by CSRS-Offset; any additional Social Security benefit to which you may be entitled based on non-federal employment will not be affected.
The offset is effective as soon as you become eligible for Social Security. For those retired by age 62, that reduction begins automatically at 62, even if they don’t apply for a Social Security benefit. Therefore, Offset employees who are retired by age 62 should apply for a Social Security benefit a few months before reaching that age. If they don’t apply for Social Security benefits, their CSRS annuity would drop anyway and not be made up by an equal amount from Social Security.
For those who retire after reaching age 62, the offset begins at retirement. Once again, they should apply for Social Security a few months before retirement so that the Social Security benefit will make up for the reduction, which will begin immediately.
Generally speaking, total benefits are either unchanged or rise slightly when the offset kicks in; it’s just that the money comes from two separate sources, the federal retirement fund and Social Security, rather than just from the retirement fund.