ANSWER: Assuming that you are disabled at the time, your annuity will start from either (1) the day after your government pay ceased; or (2) the day after you were separated from government service. If your government pay ceases and then starts up again, the resumption will cut off your right to collect disability retirement for the period during which your pay had ceased. Therefore, make sure that once your pay has ceased for any considerable time, that you do not receive any further pay from your government job before finally being separated.
Example: Peter, a government employee, applied for disability retirement and went on LWOP status for nearly a year, pending approval of his application. At his boss' urging he returned to his government job part-time for two weeks. After he won disability retirement, OPM refused to pay him his annuity for the nearly one year he had been out of work. By going back to work for the government, Peter forfeited his retroactive disability annuity for the nearly one year period. He would have been awarded the annuity for that period if he had not worked those two weeks for the government or if he had worked for a non-government employer instead.