Rinse the octopus thoroughly under cold water. Cut the body sac away from the tentacles and slice the tentacles into 5–6 cm pieces. If using pre-cleaned frozen octopus, thaw fully in the refrigerator overnight, then rinse and cut.
Place the olive oil in a large, heavy-bottomed pot or Dutch oven over medium heat. Add the diced onion and cook until softened and translucent, about 6 minutes. Add the garlic and cook for 1 minute more.
Add the octopus pieces and increase the heat to medium-high. Cook, stirring occasionally, for 5–6 minutes. The octopus will turn pink and release a significant amount of purple-tinted liquid. Do not drain it — this is the foundation of your sauce.
Pour the red wine into the pot and stir well. Let it bubble actively for 2–3 minutes, stirring to lift anything from the bottom of the pot. The alcohol will cook off and the wine will begin to reduce.
Add the crushed tomatoes, bay leaves, cinnamon stick, oregano, and black pepper. Stir to combine. Bring to a gentle boil, then reduce the heat to low. Cover and simmer for 55–60 minutes, until the octopus is very tender when pierced with a fork.
Remove the lid and increase heat slightly to medium-low. Simmer uncovered for 15 minutes, stirring occasionally, until the sauce has thickened noticeably. Taste and add salt if needed. Fish out and discard the bay leaves and cinnamon stick.
While the sauce reduces, bring a large pot of generously salted water to a boil. Cook the spaghetti or linguine per package directions, stopping 1 minute short of the recommended time. Before draining, reserve a ladleful of pasta water.
Drain the pasta and add it directly to the pot with the octopus sauce. Toss over medium heat for 1–2 minutes, letting the pasta absorb the sauce. If too thick, add a splash of the reserved pasta water. Drizzle with fresh olive oil and toss once more.
Divide between four warm bowls. Scatter fresh parsley over the top and add a final drizzle of good olive oil. Serve immediately.