What are the eligibility rules for disability retirement under FERS?
You may be eligible for disability retirement if you have completed at least 18 months of service and you are deemed totally disabled for useful and efficient service—that is, no longer able to successfully perform the critical or essential elements of the position (or other vacant positions at the same grade or pay level for which you would be qualified) even if you might be able to hold some other job. In addition: the disability must be expected to last at least one year; and your agency must certify that it is unable to accommodate your disabling medical condition in your present position and that it has considered you for any vacant position in the same agency, at the same grade or pay level, and within the same commuting area, for which you are qualified for reassignment.

This definition of disability is less strict than that used by Social Security.

An application for disability retirement must be filed before separation from service or within one year after.
Under FERS, if you apply for civil service disability retirement, you must also file for Social Security disability retirement. The Office of Personnel Management must receive a receipt or notice of approval or disapproval of disability benefits from the Social Security Administration before any disability annuity benefits can be paid. If the application for Social Security disability benefits is withdrawn for any reason, OPM will dismiss the FERS disability retirement application upon notification by the SSA.