Place onion, broccoli and butter in a 2 quart casserole. Cover. Microwave at high (100%) until broccoli is crisp tender, and onions are opaque, 5 to 6 minutes.
Add rice, corn and tomatoes. Stir in salt, pepper and oregano. Mix well. Cover. Microwave at medium high (70%) until heated through, 7 to 8 minutes.