Officer Donald O. Nielsen was assigned to the Oakland Police Department Traffic Division as a motorcycle officer. On March 22, 1951, in the late afternoon, Nielsen was returning from his residence in San Leandro after checking on his wife, who was home ill. As he approached the intersection of 149th and MacArthur Boulevard, in San Leandro, he swerved to avoid hitting a turning motorist. Nielsen lost control of the motorcycle and was thrown 52 feet from the downed machine. He sustained severe skull fractures and died three hours after the accident.
Nielsen, 28, had received his appointment to the department on February 14, 1947. He was survived by his wife (who was pregnant at the time of the accident) and two boys.