Preheat oven to 375F.
In an 8-inch-by-8-inch baking dish, add rice, black beans, corn, salsa, chicken broth, chili powder and oregano. Stir to combine.
Cut tuna into 3 pieces and submerge into liquid in the baking dish as far as possible.
Cover casserole dish tightly with foil and bake for 1 hour until rice is tender and liquid has been absorbed.
Remove from oven and sprinkle with cheese. Return to oven and bake just until cheese has melted.
Top with green onions and serve.