Preheat your oven to 400°F. Line a baking sheet with parchment paper.
In a large bowl, combine the ground chicken, breadcrumbs, Parmesan cheese, parsley, red onion, garlic, oregano, salt, pepper, and egg. Mix until well combined.
Form the mixture into meatballs, about 1 inch in diameter, and place them on the prepared baking sheet.
Drizzle the olive oil over the meatballs. Bake in the preheated oven for 20-25 minutes, or until cooked through and golden brown.
While the meatballs are baking, prepare the tzatziki sauce. In a medium bowl, mix together the Greek yogurt, grated cucumber, garlic, lemon juice, dill, salt, and pepper. Adjust seasoning to taste.
Once the meatballs are done, remove them from the oven and let them cool slightly before serving with the tzatziki sauce.