The Reagan administration, in an unusually strong retort to black African charges that the United States is in "collusion with the South African racists" said yesterday that the accusations are "serious distortions" of U.S. policy and "unhelpful contributions" to the settlement of racial conflicts in southern Africa.

In a statement read by State Department spokesman Dean Fischer the administration officially objected to a resolution adopted unanimously Saturday by the 50 members of the Organization of African Unity. It charged the United States with conspiring with South Africa to circumvent United Nations efforts to achieve independence for Namibia.

The statement also expressed U.S. displeasure at indications that Libyan strongman Muammar Qaddafi, whom the administration regards as a major inciter of international terrorism, will be elected chairman of the OAU next year. It said Libya's record of terrorism and aggression against its neighbors "hardly [qualifies] it to be the spokesman for Africa to the world."

In respect to the OAU's denunciation of an alleged "unholy alliance between Washington and Pretoria," the statement said:

"Our contacts with South Africa on that issue and on other matters of common interest should in no way serve as the basis for suggestions, such as those in the OAU resolutions, that the United States is pursuing policies supportive of South Africa's racial policies or of its continued control of Namibia. Such suggestions are untrue, unhelpful and do not advance in any way our mutual efforts in pursuit of peace and stability in southern Africa."

The exchange made clear that the administratiuon still is encountering great difficulty in overcoming black African suspicions of its policy of seeking better relations with South Africa.

The tough nature of the U.S. response to the OAU, an organization that the United States traditionally has treated with elaborate deference, also indicated that the administration may be moving away from conciliation to a harder line in dealing with the black African states.

At issue is the desire of President Reagan and Secretary of State Alexander M. Haig Jr. to end the long period of strain in U.S.-South African relations and woo that country toward closer strategic cooperation with the West.

These U.S. hopes for ending South Africa's international isolation turn on efforts to find a formula for granting independence to Namibia, a predominantly black territory that has been controlled by South Africa since the end of World War I.

The administration has endorsed a three-year-old plan for bringing about independence under U.N. auspices. But, in order to overcome South African objections that the U.N. plan would turn control of Namibia over to the Southwest Africa People's Organization (SWAPO -- a group the South Africans regard as communist-dominated -- the administration has proposed some new approaches to dealing with the problem.

In broad outline, these call for negotiations in advance of independence that would determine Namibia's form of government and ensure a voice for its other political parties.

Although that idea has been endorsed by America's principal allies, it is widely regarded in black Africa as a device for denying power to SWAPO and ensuring continued, indirect South African control over the territory through puppet political groups responsive to Pretoria's bidding.

In respect to Qaddafi, the U.S. statement yesterday noted that the OAU's 1982 meeting will be held in the Libyan capital of Tripoli and that traditionally the host head of government becomes OAU chairman for the ensuing war.

If that happens, the statement added, "we would look upon it with deep regret. . . . Libya's support for international terrorism, its intervention in the affairs of neighboring states (including its incursion into Chad), and its assassination campaign against Libyan dissidents abroad hardly qualify it to be the spokesman for Africa to the world."

The State Department also confirmed yesterday that Haig will meet the foreign ministers of Canada, Mexico and Venezuela in Nassau, the Bahamas, on July 11 to discuss cooperative plans for increased economic aid for the Caribbean basin.

From Nassau, Haig will fly to New York to head the U.S. delegation at a special U.N. conference beginning July 13 to seek ways of resolving the civil war in Kampuchea.