In a large bowl, combine the cooked rice, drained tuna, condensed cream of mushroom soup, milk, diced onion, frozen peas, garlic powder, onion powder, dried oregano, salt, and black pepper. Stir until well mixed.

Transfer the mixture to a greased baking dish. In a small bowl, mix together the butter and cracker crumbs.

Sprinkle the shredded cheddar cheese evenly over the top of the rice mixture, then sprinkle the cracker crumbs over the cheese.

Bake in the preheated oven for 30 to 40 minutes, or until the casserole is bubbly and the top is golden. Let it cool for a few minutes before serving.