Place the chicken thighs and potatoes in a large mixing bowl. Drizzle the 3 TBSP of olive oil over the chicken and potatoes then sprinkle on 1 tsp salt and 1/2 tsp freshly ground black pepper. Toss the chicken and potatoes together until everything is evenly coated.
Spread the potatoes out on one side of a large rimmed baking sheet then place the chicken thighs skin-side up on the other side of the baking sheet.
Bake in the preheated oven until chicken is cooked through and potatoes are crisp, about 40 minutes.
Transfer the chicken and potatoes to a serving dish. Drizzle the lemon juice and the remaining 1 TBSP of olive oil over just the potatoes then sprinkle the oregano over both the chicken and potatoes.