Place the octopus in a pot filled with water and boil it for about 1 hour. Check with a fork to see if it is done, it must be tender. Drain and allow to cool on a plate. Cook the potatoes in the same water and pot where you cooked the octopus. When done peel them and cut them into pieces. In the meantime clean the octopus and cut it into small pieces. Put it in a bowl and add the lemon juice, finely chopped garlic, the Polli black olives, the parsley and the potatoes. Dress with oil and mix all the ingredients well. Add salt to taste.