WASHINGTON (AP) _ Bob Mohan, host of the ''Sound-Off'' radio talk show in Atlanta, was sitting in his office one day when a White House aide called offering an interview with President Reagan. He was surprised, he said, but became even more startled over the controversy sparked by Reagan's comments on South Africa.

Mohan said by telephone Wednesday that he didn't realize Reagan was making news when he described South Africa's white regime as ''a reformist administration'' and that it had ''eliminated the segregation we once had in our country ....''

White House spokesman Larry Speakes said later that Reagan didn't really mean to say, nor did he believe, that all segregation had been ended in South Africa. He was talking about the desegregation of some restaurants and other facilities in some of the larger cities, Speakes said.

Mohan said the interview didn't have much impact initially on his listeners either. He said there was just one call Monday night - saying Reagan was ''naive'' about South Africa - after the seven-minute interview was aired.

But after the interview became front-page news across the country, Mohan said he invited calls Tuesday night on whether people thought the press had ''overblown'' Reagan's comments.

He said the telephones wrang constantly during the two-hour night-time show, and that all but a handful of the calls dealt with Reagan's comments.

The 25 calls on the issue were three-to-one in sympathy with the president and felt ''the press was carping at him because they had nothing else to do.''

''They (the callers) feel that the president probably knows what he is doing,'' Mohan said.

''While he may have made an unfortunate statement regarding the situation in South Africa, they say, 'Hey,' everybody is entitled to mistakes. (The Rev. Jerry) Falwell, has made them, (Former President) Carter, everybody has. They felt it was unfair for all the newspapers to zero in on that one statement.''

Mohan, 48, has been host of the talk show on radio station WSB for the past three years. With the station's 50,000-watt transmitter, he said the show is heard in 36 states during night-time hours.

Mohan said the interview was offered to him unexpectedly by the White House about a month ago. ''They obviously wanted to give someone an outlet for it in the South and picked me for it,'' he said. ''I am eternally grateful.''

He said the White House wanted to know ahead of time what subjects Mohan would cover in the interview, but that the specifics of the questions weren't discussed ahead of time. Most of the questions concerned South Africa, with some talk of Reagan's tax reform proposal.

Mohan said his listening audience generally reflects the political make-up of the region - ''very Democratic, but very conservative ... wholeheartedly for Reagan.''

South Africa is a popular subject on the talk show, he said, and there has been both criticism and praise of Reagan's controversial policy of ''constructive engagement.''

Mohan said he also has been critical of contructive engagement because ''I don't think it's working.'' But he said he opposes sanctions and thinks South Africans ought to be able to settle the issue themselves without outside interference.

Mohan said he questioned Reagan's use of the word ''reformist'' to describe the South African government of President P.W. Botha. ''That word got him into trouble,'' said Mohan. ''If he had said the regime was attempting change, it would have been better. Reformist is a heavy word.''

But he said after he saw the reaction in the nation's newspapers to the interview, he was surprised by it and concluded, ''I simply wouldn't want to be president of the United States, in no way, shape or form.

''I'm a Reagan supporter on most issues, but I just don't know what the man can do in South Africa.''