While the octopus is boiling, prepare a brazier or other heavy, oven-proof pot with a lid (a Dutch oven works fine) by lining it with a nest of herbs and spices.
What herbs? I cook Mediterranean food, so I use rosemary, fennel fronds, bay leaves, fig leaves, fresh oregano -- something that will fit the ultimate recipe I am using. Think about the end dish and incorporate complimentary flavors here.
If you cook in another cusine, add appropriate herbs and spices: Lemon grass, ginger, Thai basil and shallot is an excellent choice. Remember that the octopus meat will absorb the flavors you give it.
Heat the oven to 200 degrees.
Take the octopus from the boiling water onto a chopping block: Chop off its head -- there's not much good meat in there anyway, and what there is is incredibly gelatinous. You need not do this with the baby ones because they typically come cleaned with only the good meat from the head left on. For some reason large octopi tend to be left whole.
Nestle the octopus' legs onto its nest of herbs in your pot. Close the lid and pop it in the oven. How long?
- For baby octopi, try them after 2 hours.
- For small ones, check after 3-4 hours.
- For the monsters, don't even bother checking for at least 5 hours.