A heavy saucepan, 4-quart capacity, with a cover A heavy-bottomed skillet or saute pan, 12-inch diameter or larger
Set the saucepan over low heat. Pour in 6 tablespoons of the olive oil. Scatter the onion in the bottom of the pan and add the garlic. Nestle the octopus on top of the onions. Scatter the olives over the octopus. Cover and let the octopus stew in its own juices for 1 hour.
Uncover the octopus, there should be a lot of liquid from the octopus in the pan. Cover and continue to stew over low heat until the octopus is very tender, about 45 minutes to 1 hour and 15 minutes more, depending on the size of the octopus. To test for doneness, stick a fork in the thickest part of the octopus, it is done if the fork slides out easily.
Remove the octopus from the pot and let cool slightly. Cut the octopus into 3/4-inch chunks, skin and all. Put the octopus chunks in the skillet and pour in the contents of the saucepan as well. Taste the sauce for salt (the octopus can be very salty) and add the salt, if needed. Get the sauce simmering rapidly over medium heat and stir in the parsley. You should have 2 cups or so of braising juices in the skillet. If there's much more than that, boil to reduce the sauce