The Rev. Canon Oliver B. Garver Jr. was elected suffragan bishop of the Episcopal Diocese of Los Angeles at a convention at St. John's Church. It was the first time in 21 years that such an election has been held. The suffragan is an assistant to the diocesan bishop, who is chief pastor and administrator of the jurisdiction. The Rt. Rev. Robert C. Rusak is the present diocesan bishop. Garver, 59, of Los Angeles is chairman of the Advocates, a support group for the County Human Relations Commission, vice president of the Southern California Ecumenical Council, and has served on the Governor's Commission on Refugees.