Operation Olympic was just one-part of Operation Downfall - the planned occupation of Japan. Operation Olympic was chronologically the first part of the plan - Operation Coronet would come second. Olympic was supported by the likes of Douglas MacArthur who favoured a massive amphibious landing on Japan as opposed to a blockade/bombing strategy favoured by the navy.
Operation Olympic was the code-name for a planned landing in Kyushu - the furthest main island in the south. This island was one of the few places in the whole of Japan that could sustain an amphibious landing. The actual invasion was planned to start on November 1st, 1945, with three landings at three different beaches. The three targeted beaches were at Miyazaki, Ariake and Kushikino. The Japanese had realised that Kyushu would be a prime landing point for any invasion and had set up strong defences at Ariake as it had a good harbour there. Once an amphibious landing had occurred, the Americans planned to move inland but only for about one-third of the island. Airbases would then be set-up to support Operation Coronet.
For Coronet to be successful, Olympic would have to be. Therefore, some military figures were quite prepared to contemplate the use of poisonous gas against the Japanese on the beaches of Kyushu - especially if the defenders moved into the caves near the beaches - a tactic America had experienced before. All the evidence suggests that the Japanese planned to repel the invading force and drive it back into the sea. The Homeland Defence Force had the potential to run into many thousands in the area and the kamikazes had shown the Allies that the Japanese were quite willing to die for the emperor and Japan. For this reason, the Americans predicted large losses at Kyushu before one-third of the island had been captured. The Chiefs-of-Staff estimated that a 90-day campaign in Kyushu could cost as many as 450,000 casualties, including over 100,000 dead.
As it was, the attack never took place as President Truman authorised the use of the atomic bomb on Hiroshima and Nagasaki on August 6th and 9th respectively. Japan formally surrendered on September 2nd 1945.