Combine olive oil, lemon juice, lemon zest, garlic, oregano, salt, and black pepper in a bowl. Place chicken breasts into a 9x13-inch baking dish. Brush lemon juice mixture over chicken.
Place potatoes, red bell pepper strips, red onion, and lemon slices in a bowl, pour remaining lemon juice mixture over vegetables, and toss to coat. Arrange vegetables and lemon slices around chicken breasts in baking dish.
Bake in the preheated oven for 30 minutes; brush chicken and vegetables with pan drippings. Continue baking until chicken is browned, the juices run clear, and an instant-read meat thermometer inserted into the thickest part of a breast reads at least 160 degrees F (70 degrees C), about 30 more minutes.