CSRS Offset is the Civil Service Retirement System with Social Security Offset. It is the same as CSRS, except that is coordinated with Social Security.
CSRS Offset was created in 1987 and generally applies to employees who had a break in Federal service after 1983 that lasted longer than 1 year and had at least 5 years of civilian service as of January 1, 1987. It also applies to employees who were hired into a civilian job before 1984, but did not acquire retirement coverage until after 1984 and had at least 5 years of service as of January 1, 1987.
CSRS Offset employees are covered by both CSRS and Social Security. You earn retirement credit under CSRS, while also earning credits under Social Security. When you retire from the Government, your retirement benefit is computed in the same way that CSRS benefits are computed. However, when you become eligible for Social Security benefits (usually at age 62), your CSRS retirement benefit is reduced, or offset, by the value of the Social Security benefit you earned while working for the Government.
The amount CSRS Offset employees pay for retirement the same amount that CSRS employees pay, however it is reduced, or offset, by Social Security taxes (6.2 % of pay). Agencies contribute a set amount (7% for most employees) to CSRS Offset.
Just like CSRS employees, CSRS Offset employees are also are allowed to participate in the Thrift Savings Plan and currently may contribute up to an $18,000 annual limit for 2015, without a Government contribution.