Wash the octopus thoroughly and pound it with a mallet; peel and wash the vegetables.
Fill an almost full pan with water, put it on the fire and bring to a boil. When the water boils, place the prepared octopus in the pan and add the celery stalk, peeled carrots, bay leaf and garlic. Cook the octopus for 30 minutes until tender.
Remove the octopus from the pan, rinse with cold water and let cool. Then cut into small pieces and place them in a marinating container. In the same bowl, add olive oil, juice of one lemon, chopped parsley and add a little salt. Cover with a lid or film and refrigerate for 8 hours.
Serve the finished marinated octopus, drizzled with olive oil and sprinkled with oregano; you can also add green olives.