OPM processed 10,955 claims last month, 1,455 more than its expected output under a revised comprehensive strategic plan for addressing the backlog.
Overtime for retirement services personnel was suspended at the end of April because of spending cuts from sequestration, according to the report.
Federal News Radio reported a year-end budget review allowed the office to reinstate limited overtime.
The agency hoped to eliminate claims backlog by the end of summer 2013 but now expects to clear up pending inventory by summer 2014, the station reports.
Nearly 90,600 federal workers have applied for retirement this year, more than 25 percent above the levels recorded in 2012.