Rinse the mussels under cold water, scrub the shells, and remove the beards. Discard any mussels that are cracked or remain open.
In a large pot over medium heat, add olive oil and the whole garlic cloves. Cook until lightly golden and fragrant, about 1 minute.
Add the mussels, white wine, and a whole parsley stem. Cover and steam for 3–4 minutes, until the mussels open.
Remove the mussels and discard any that remain closed.
Strain and reserve all of the cooking liquid, then discard the garlic and parsley.
In a clean pot, add olive oil, chopped garlic, and red pepper flakes. Let infuse gently for 1–2 minutes.
Add the crushed tomatoes, oregano, salt, and pepper. Simmer for 5–6 minutes.
Pour in the reserved mussel liquid and let simmer for 10–15 minutes.
Return the mussels to the pot, add fresh parsley, and cook for 2–3 minutes to bring everything together.
Serve immediately with crusty bread.