Stir together the paprika, kosher salt, garlic powder, onion powder, cayenne pepper, ground black pepper, ground white pepper, dried thyme, and dried oregano in a small bowl.
Pat the mahi mahi fillets dry; melt the butter, then brush the fillets on both sides with it and coat evenly with the spice blend, pressing so it adheres.
Heat a large cast-iron skillet over high heat until very hot, 4–5 minutes, until you see a faint wisp of smoke.
Add the vegetable oil and swirl to coat the pan.
Lay the seasoned fish in the skillet and cook without moving until a dark, charred crust forms, 2–3 minutes. Flip and cook 2–3 minutes more, until the fish flakes with a fork and the center reaches 130–135°F; reduce heat slightly and add 1–2 minutes if the fillets are very thick.
Transfer to a plate and rest 2 minutes to finish cooking gently and set the juices.