Because of substantial, last-minute reductions in benefits that will be paid next year to federal workers and family members seeking abortions, dental or mental health treatment the government will announce today a delay in the open season enrollment period -- originally scheduled to run from Nov. 10 to Dec. 5 -- for the 9.2 million people covered by Uncle Sam's office health plans.

Many of the 100-plus carriers in the FEHB (federal employee health benefits) program are scrambling to meet a Friday deadline set by the Office of Personnel Management. OPM told them to keep premium increases to a minimum next year (around 10 or 11 percent) by trimming or eliminating benefits they pay now for certain kinds of medical treatment. Insurance companies that do not meet the Friday deadline, or that submit 1982 premium and benefit proposals that do not please the Office of Personnel Management, will be dropped from the program for next year.

Normally federal and postal workers, and retired U.S. workers, have a three-week open season period beginning in early November. During open season they may shop for better insurance deals, expand or cut their coverage or look for plans with lower premiums. Because of the confusion this year, and OPM's avowed desire to minimize the premium rise by cutting benefits, federal officials had seriously considered canceling the open season altogether. That would have locked workers and retirees into their current health insurance plans, no matter how high premiums went up or how much benefits were cut for 1982.

Officials now say the "open season" will be held -- probably this year. But it will come later than the usual change-your-plans time period..

Rates and benefits in the FEHB are a big deal. It is the biggest "company" program in the nation -- therefore a trend setter for the private sector. In metro Washington the FEHB is the primary payer of doctor and hospital bills for nearly half the population.

Details on the 1982 premium and benefit levels will be announced by OPM the first week in October. Workers and retirees will have an open season period to make changes but not when they expected it.

General Services Administration will announce some top-level appointments today, when its 11 regional administrators and commissioners of various GSA components meet. Rumors of shakeups and layoffs have been sweeping the housekeeping agency, but if there are any coming they are not expected to be announced today. Office of Management and Budget's deputy director Ed Harper will keynote the session. New Hampshire businessman Gerald Carmen, who has been running GSA for the past three months, has a number of studies and audits going to figure out ways to tighten up GSA's multimillion-dollar purchasing and contracting programs that have made GSA a four-letter word with the press, public and politicians. He is getting high marks, so far, from White House types who say they are anxious to remove the smell of pork (as in barrel) from the agency.