Line a rimmed baking sheet with foil or parchment paper.
Pat dry the thighs and place on the prepared baking sheet. Set aside.
In a small bowl, stir together the garlic and onion powder, oregano, salt, and pepper.
Under the skin of each thigh, sprinkle on some of the spice mixture (about a ½ teaspoon) and tuck in a lemon slice. Replace the skin and tuck any loose skin under the bottom of the thigh. Repeat with all the thighs.
Divide the olive oil among the thighs and rub to lightly coat. Sprinkle on some of the spice mixture (about ½ teaspooand spread with clean hands until thigh is mostly covered.
Cover with plastic wrap and let sit in the refrigerator for 1 hour.
Thirty minutes before removing the chicken, preheat the oven to 400°F.
Bake chicken for 40 – 50 minutes, until the thickest part of the thigh measures 165°F. If the skin hasn’t browned, place under the broiler for 1 – 2 minutes. Let sit for 10 minutes before serving.