In a large skillet over high heat, warm the oil. Season the chicken with salt and pepper, then brown on all sides, about 6 minutes total. Reduce the heat to medium, add the onion and oregano, and cook, stirring constantly until fragrant, 1 to 2 minutes. Add the reserved lemon juice and lemon slices and cook, scraping any browned bits from the skillet bottom with a wooden spoon, until the liquid has evaporated, 1 to 2 minutes.