On a flat work surface or cutting board, pound the whole octopus using a skillet, a meat tenderizer, or other heavy object. Pounding will help tenderize the octopus flesh. Then, using kitchen shears or a sharp knife, cut and remove the tentacles of the octopus, making sure to cut up and around the webbing. Discard the octopus head.
Bring a large pot of salted water to a boil. Add the octopus tentacles, reduce the heat, and simmer for 1 hour. Check the octopus with a fork to check for doneness. Once the octopus is tender, drain and cut the tentacles into 1 1/2 to 2-inch pieces and set aside.
In a large deep skillet, heat 1/4 cup olive oil over medium heat. Add garlic and red pepper flakes. Cook for about 4 minutes, or until the mixture is fragrant. Add the crushed tomatoes and salt, and then allow the sauce to simmer for 10 to 15 minutes. Stir in the olives, and check if more salt is needed. When the octopus is ready, add to the sauce.
Meanwhile, cook the pasta in a large pot of boiling salted water until it is al dente. Drain and place in a large bowl. Add the octopus and sauce to bowl and toss together. Top the pasta with fresh parsley and serve.