Fill a 5.5-gallon pot with 3 gallons of water and bring it to boiling. Prepare an ice bath in a large bowl. Blanch the octopus for 2-3 minutes until its tentacles contract and curl. Transfer to the ice bath.
Slice the cold octopus in half making two flat portions. Vacuum pack each portion with 1/2 tbsp of EVOO. Refrigerate until ready to cook.
Prepare sous vide. The temperature for the water bath is 171 °F / 77 °C. Cook for 5 hours. Regrigerate vacuum packed until ready to serve. Reheat before serving.