Preheat the oven to 400F/200C. Prepare a baking dish with slightly scrunched foil in it (to help the mussels be able to sit level).
Scrub the mussels, removing any dirt and hairy bit, clean and discard any that are broken.
Put the mussels in a pot with the water, cover and put on a medium-high heat for a couple minutes until they open up. Remove form the heat.
Meanwhile, mix together the breadcrumbs, parmesan, parsley, garlic, oregano and olive oil in a small bowl.
When the mussels have steamed, remove one side of the shell and use it to loosen the mussel from the shell, if still attached. Sit the mussel on one side of the shell and set it level on the foil in baking dish. Repeat with the rest of the mussels.
Spoon/press some of the breadcrumb mixture on top of each mussel, trying to cover the mussel itself as far as possible. If you like, put a little of the mussel cooking juice in the bottom of the shell to help keep the mussel moist.
Bake the mussels for approx 7-10 minutes until gently golden on top. Serve warm or room temp (don't re-heat them as they will likely go dry and/or tough).
Note this recipe gives a relatively light coating of breadcrumbs which is a nice balance of crumb to mussel, but if you prefer more crumbs, you can make more of the topping mixture.