Heat a skillet over medium heat and add the oil, followed by the spinach. Sprinkle the oregano and garlic over the spinach. Toss gently, and cook until the spinach is very tender, and most of the water has cooked out, about 10 minutes. Let the spinach cool to room temperature, then chop it finely. Fold the spinach into the yogurt, along with the lemon juice. Season with salt and pepper, and serve.