An Overseas National Douglas C-54D (N79992, msn 10832) took off from Oakland Airport, California at 08:20 on a captain’s instrument competency check. At 09:23 the flight was given clearance to make a practice range approach and to remain above 1500 feet. At 09:35 a California Eastern DC-4 (N4002B, msn 10474) took off from Oakland for a similar check-flight. The two aircraft approached Oakland Range (located 0,2 mile NE of runway 15) at 3000 feet; the Overseas National aircraft on a 124ø heading at a speed of 140 mph, the California Eastern plane on a 75ø heading at a speed of 170 mph. Both aircraft continued until they collided. The empennage of the ONA aircraft was torn off and the plane crashed on a highway and caught fire. Eleven people on the ground were injured. The California Eastern aircraft was able to make an emergency landing at San Francisco Airport. PROBABLE CAUSE: “The failure of the Overseas safety pilot and/or his observer to observe and so avoid the other aircraft and the failure of California’s safety pilot to carry a qualified observer aboard the aircraft to insure adequate field of vision.”