The federal Office of Personnel Management yesterday approved Blue Cross-Blue Shield's plan to refund $289 million in health insurance premiums to U.S. workers and retirees and to give a $465 million refund to the federal health insurance program that covers half the people in the Washington area.

OPM said the tax-free payments -- from $18 to $374 -- will go as soon as possible to 750,000 employes who are enrolled with the nation's largest health company. It said the White House will seek legislation that would permit identical refunds to 750,000 federal retirees who have Blue Cross-Blue Shield policies.

President Reagan approved the payments last week, OPM said, before the Justice Department issued a legal opinion saying refunds to employes were legal but special legislation was needed before retirees could get the payments. Workers and retirees get the same coverage and pay the same premiums, but because of a quirk in the federal health insurance law dealing with refunds and premium costs, the measure refers only to "employes" and not "annuitants."

About 200,000 people here are due the refunds. The decision was announced late yesterday and caught insurance company officials and interested members of Congress off guard. One Republican member of Congress had just sent a letter to OPM demanding a decision on the payments when his staff learned of the decision.

Blue Cross-Blue Shield proposed the giant rebate in May. It said costs to the plan were down more than 22 percent because policyholders' claims were down or they were paying a bigger share of their medical and hospital bills because of cost-containment features imposed by the Reagan administration.

A company spokesman praised the OPM decision but said he wished it had come sooner. Although many of the more than 200 plans in the federal health insurance program have experienced similar declines in use, no other federal health insurer has announced rebate plans. However, OPM said yesterday some of the plans "may be following the . . . action and offer cash rebates." About 45 percent of federal policyholders have Blue Cross-Blue Shield insurance. Legislation to give retirees the rebates is expected to move quickly through Congress.

Under the plan, white-collar civil servants with low-option coverage will get refunds of $73. Those with high-option family plans will get $374. Because the government pays a bigger share of their health premiums, rebates for postal employes will range from $18 for single policyholders to $140 for those with family high-option coverage.

Blue Cross-Blue Shield has set up a toll-free hot line, (800) 253-0123, for refund information.