Washington writer Grant Oliphant has become the editor of a new magazine, American Politics, which will begin monthly publication after the first of the year. The nonpartisan, slick publication, in addition to covering the political spectrum, will present the political opinions of nonpolitical people like musicians and businessmen . . .

In the continuing storms of violence in South Africa, Humanitas International, led by activist singer Joan Baez, has written an open letter to Nobel Peace Laureate Bishop Desmond Tutu, supporting his commitment to nonviolence. Among the international list of cosigners was Lech Walesa, leader of the outlawed Solidarity union; United Farm Workers President Cesar Chavez, and Atlanta Mayor Andy Young . . .

Georgetown's Four Seasons Hotel is listed in International Investor magazine as the best hotel in the Americas and 16th in the world. The Madison Hotel, the only other Washington hotel on the list, was 9th in the Americas and 35th in the world. To compile the list, the magazine polled some 80 senior bankers who average 74 nights a year in hotels . . .

The bicoastal divorce battle between those two People magazine lovebirds, John DeLorean and Cristina Ferrare, had both sides claiming victory yesterday when a New Jersey judge made a tentative decision to recognize a California prenuptial agreement, even though it will reduce Ferrare's property settlement. Apparently they had earlier guessed it might not last forever . . .

Prince Andrew, who passes as a British royal playboy with his well-publicized liaisons, has yet to meet the woman of his dreams, but he told a radio interviewer he'll know when it happens. "It will happen but who knows when? All I know is that if I do find somebody it will come like a lightning bolt and you are going to know it there and then" . . .