Combine oil, salt, pepper, oregano, and garlic powder; brush on lamb. Place lamb, bone ends up, in a shallow roasting pan. Insert meat thermometer, if desired.
Bake, uncovered, at 400° until desired degree of doneness: about 35 minutes or 140° (rare); about 55 minutes or 160° (medium); about 1 hour and 10 minutes or 170° (well done).
Transfer lamb to a warm serving platter, reserving pan drippings. Sprinkle with parsley. Let lamb stand 10 minutes before slicing.
Add wine to pan drippings in roasting pan. Cook over low heat, stirring with a wooden spoon to loosen browned crumbs. Add butter; simmer 4 to 5 minutes, stirring occasionally. Serve sauce with lamb.