About twenty minutes after DEPARTURE, the pilot advised Coffs Tower he was returning after observing a reduced engine oil pressure indication. Soon after, he reported oil fumes in the cockpit and requested a priority landing from a straight in approach to runway 03. The aircraft touched down heavily on the left main and tail wheel, collapsing the main landing gear and shattering the wooden propeller. The pilot reported the oil leak had emanated from an incorrectly assembled firewall fitting on the oil line to the oil pressure indicator. That fitting had been installed prior to the present owner purchasing the aircraft. The main gear is normally held in the locked down position by two mechanical latches. When the aircraft touched down heavily on the left wheel, one mechanical latch had slipped from the locked position. The other latch had not engaged due to misrigging.