In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set a side.
Pour the buttermilk in a large measuring cup, whisk in the oil and egg and mix to combine.
Add the liquid mix into the dry mix and stir just until there are no flour streaks. Gently fold in the feta mixture into the batter.
Evenly divide the batter between the muffin liners and baker for 18-20 minutes, or until a tooth pick comes out clean from the center of the muffins.