Prepare the octopus. Pour water into a large saucepan, squeeze the juice of half a lemon into it and leave the lemon in the water, pour in a glass of wine. Put the saucepan on the fire, put the octopus in it, cover with a lid and let the water boil. Make the fire low and cook the shellfish for 2 hours. Turn off the stove, let the octopus cool without removing it from the saucepan. Then separate the body from the head, remove the skin and cut into pieces.
Prepare the marinade. In a deep bowl, mix olive oil, juice of the second half of the lemon and oregano. Marinate the octopus for 120 minutes.
Start frying. Grill the octopus pieces for 3 minutes on top and bottom, add rosemary and sprinkle with lemon juice when ready.