The U.S. Export-Import Bank has extended $85 million in credits to finance an offshore oil development project in Angola, despite the openly voiced opposition of the Reagan administration to that African nation's Marxist government.

The loan agreement, the first Ex-Im financing provided to Angola since Portuguese colonial rule collapsed there in 1974, was signed in Washington on Wednesday by Ex-Im bank officials and Jose Carlos Victor de Carvalho, governor of Banco Nacional de Angola, Angola's central bank. It provides U.S. government loans for the further development of Angolan oil fields now being jointly undertaken by a Gulf Oil Corp, subsidiary and Sonamgol, Angola's state-run oil company.

State Department officials said the decision to proceed with the long-delayed loan agreement marked no departure in U.S. policy toward Angola, but acknowledged that the growing American financial involvement there contrasts with the impression of implacable opposition, voiced by Secretary of State Alexander M. Haig Jr. at confirmation hearings earlier this year, to the Marxist takeover of Angola six years ago.

The funds from the Ex-Im Bank, a federally funded institution that lends money at special rates to foreign borrowers to buy American goods, will be used to help finance a $160 million project designed to double oil procuction from two main wells off Cabinda on Angola's coast to about 200,000 barrels daily by 1985.

Extending the credits, approved under the Carter administration in April 1980, was delayed, Ex-Im officials said, because of the "complexities" of arranging further funding from commercial banks to complete the investment package.The last obstacle apparently was overcome in Paris on June 30, when a commercial syndicate led by Morgan Guarantee Trust agreed to advance approximately $50 million in private loans.

The Reagan administration has said it will refuse to recognize the Angolan government as long as an estimated 20,000 Cuban troops sent there in 1976 remain. Angola also is believed to recieve military and economic assistance from the Soviet Union.

In a move widely interpreted as an attempt to help hold the line against communist expansion, the administration has asked Congress to repeal the Clark amendment, which outlaws aid to Angolan rebels seeking to over-throw the government in Luanda. Haig had said in his confirmation hearings that the U.S. failure to support UNITA, the major group in Angola, helped bring about the victory of the Movement for the Popular Liberation of Angola in the 1976 civil war.

Haig and other officials have said that they want the amendment repealed only because they view it as unacceptable restraint on presidental authority.

A Senate staff aide who has followed U.S. policy on Angola suggested that the loan agreement may reflect a moderation of the administration's views.

The Angolan ambassador to the United Nations, Elisio de Figueiredo, met in Washington Wednesday with Chester Crocker, assistant secretary of state for African affairs. The State Department called the visit an "infrequent" courtesy call.

Gulf Oil and other major American oil companies that have a stake in oil development in Angola have been encouraging Washington to soften its policy toward Angola's Marxist government. During congressional hearings last September, Melvin J. Hill, president of Gulf Oil Exploration and Production, a Gulf Oil unit, said that the Angolan government has "been responsive and supportive as a business partner." Gulf Oil, Hill said, had "encountered no ideological or discriminatory problems of any significance."

But a U.S. State Department official emphasized that "the administration could not envisage the establishment of diplomatic relations with Angola as long as Cuban troops remain there." He suggested, however, that the government in Luanda has showing a willingness to cooperate on negotiations involving the disposition of Namibia, the trust territory controlled by South Africa. Angola has supported Namibian rebels trying to oust South African rule.

He added that "the [U.S.] commercial relationship continues with Angola -- there are several very large American firms active" there. That relationship is not likely to change, he suggested, since "the administration has said repeatedly that there has been no decision to provide aid to UNITA."

Rep. Howard Wolpe (D-Mich.), chairman of the House subcommittee on Africa, who will be leading a congressional delegation to Angola and other African countries in August, said: "There is a certain irony in that while we are facilitating private sector American investment, which makes sense from the standpoint of American business interests and energy requirements, we still maintain a posture of [diplomatic] nonrecognition."

He said of the move to repeal the Clark amendment: "Angola finds it very threatening" and it ultimately could affect U.S. relations with other African nations.