DIED. Oliver Tambo, 75, dignified former president of the African National Congress; of a stroke; in Johannesburg. After opening Johannesburg's first black law firm, Tambo and Nelson Mandela helped found the A.N.C. Youth League in 1944 and battled apartheid in the streets--as well as the courts. After the A.N.C. was banned and Mandela imprisoned, Tambo led the struggle in exile, working for diplomatic and economic sanctions against South Africa and setting up guerrilla units in neighboring black states. Debilitated by a stroke since his return in December 1990, he was replaced as president in 1991 by Mandela.