Heat the oven to 375°F. Lightly coat 8 cups of a standard muffin tin with the olive oil.
In a large mixing bowl, combine the ground turkey, spinach, feta, breadcrumbs, egg, red onion, garlic, oregano, and black pepper. Mix gently until the ingredients are evenly distributed, taking care not to compact the mixture.
Divide the mixture among the 8 prepared muffin cups. Press it in lightly so each cup is filled without packing the turkey too firmly.
Bake until the muffins are cooked through and their centers reach 165°F on an instant-read thermometer, about 20 to 25 minutes.
Let the muffins rest in the tin for 5 minutes, then carefully lift them out and serve warm.