Heat the oven to 400°F (200°C). Line two rimmed baking sheets with parchment and brush lightly with oil.
In a large bowl stir together the bread crumbs and milk. Let stand 5 minutes until the crumbs have softened into a paste.
Add the egg, Parmesan, grated onion with its juice, garlic, parsley, oregano, salt, pepper and pepper flakes. Mix well.
Add the turkey and mix with your hands just until evenly combined. Overmixing makes meatballs tough.
With damp hands or a small scoop, shape into 1 1/2-inch balls, about 2 tablespoons each. Space them on the sheets.
Bake for 16–20 minutes, rotating the pans halfway, until lightly browned and cooked through to 165°F (74°C) in the center.
Serve right away in sauce or let cool completely for storing.