Office of Management and Budget's decision to backdate President Reagan's federal hiring freeze to Nov. 5 has thrown the bureaucracy, and would-be federal workers, into as tailspin. The retroactivity order was issued Saturday. It says that persons offered federal jobs after Nov. 5 who are not already on the payroll cannot be hired. It is expected to result in a rash of lawsuits from federal unions, and individuals who may be denied jobs they thought they had.

The freeze covers all executive branch agenices and jobs, except those involved in direct health care, safety and the protection of property. Agencies seeking exemptions from the freeze will have to apply directly to the OMB. Individuals who have been blocked from jobs they had been promised cannot go directly to OMB. The agency that wants to hire them must do that.

Also exempted from the hiring freeze are agencies of congress, and the U.S. Postal Service, political appointees and the filling of noncareer jobs in the Senior Executive Service.

Federal workers who were planning to move to another job in another agency probably will be blocked by the freeze, unless that transfer is as a result of actions ordered by the congress or White House.

Reagan Administration officials fully expect to be sued -- by disappointed job-seekers and federal unions -- for extending the freeze back to Nov. 5. Meantime, hundreds of people promised jobs since Nov. 5 who did not get on the payroll are probably going to have to wait to become civil servants.