Place the mustard sauce, yogurt, balsamic vinegar, sugar and dried oregano in a small bowl; mix well and set aside.
Wash the tilapia fillets and pat dry. Season both sides with salt and black pepper.
Preheat oven to 390 degrees F. Grease the bottom of a baking dish lightly with cooking spray or canola oil. Place the seasoned tilapia fillets in the baking dish. Set the small pieces of butter on top.
Place on the middle rack of the oven and bake until cooked through, about 17-20 minutes. When they are done, remove from oven. Place on a platter, spoon the sauce over them and sprinkle with dried oregano leaves. Serve immediately.