Government workers and retirees would pay a smaller portion of their health insurance premiums under legislation the House Post Office-Civil Service Committee is expected to approve Thursday.

The bipartisan bill, introduced by Rep. Mary Rose Oakar (D-Ohio), would raise the limit on government contributions to federal health premiums, currently 75 percent, and would require the government to hold open-enrollment periods each year when workers and retirees could shop around for better insurance deals.

The government now pays about 61 percent of the premiums for white-collar federal workers and up to 75 percent for postal employes.

Oakar's bill, cosponsored by Rep. Frank Wolf (R-Va.), would guarantee that retirees get the same health premium refunds being offered to current subscribers by 11 health plans.

Under the current federal health insurance law, only active duty workers can get refunds. The Reagan administration supports the proposed change in the law that would entitle workers and retirees to the refunds, ranging from $18 to more than $400.

Plans offering the refunds include Blue Cross-Blue Shield, National Association of Letter Carriers, Foreign Service, Government Employees Health Association, Government Employees Benefit Association and Aetna.

More than 2 million subscribers, including about 300,000 in the Washington area, are due refunds.

Oakar's bill cleared the compensation subcommittee Friday. The Post Office-Civil Service Committee will take it up shortly.

Refunds proposed by Blue Cross-Blue Shield for current policyholders are being challenged in court by people who subscribed to the health plan in 1983 and 1984 but not this year. They believe they are entitled to part of the refunds, which were made possible because fewer insurance claims have been made and more people are assuming a greater share of their health care costs.

If the court rules that 1983 and 1984 subscribers are entitled to a share of the refunds, it will trim the proposed refund amounts due to 1985 subscribers.