Bring a large pot of water to a boil. Add the octopus to the water, two at a time. When the water returns to a boil, remove the octopus and set aside to cool. Repeat with the remaining octopus.
Bring a large pot of water to a boil and add salt. Add the octopus and simmer until the meat is tender, 20 to 40 minutes. Drain and chill under cold running water. Cut off any non-fleshy parts including the "beak" and discard. Cut into bite-size pieces.
Put the octopus pieces in a large bowl and add the celery, lemon juice, olive oil, parsley, basil, red pepper flakes, olives, garlic and salt and pepper to taste. Toss to blend and let stand 1 hour.