Heat oil in a large skillet over medium heat. Cook and stir ginger in the hot oil until slightly softened and brown, 5 to 7 minutes. Add catfish fillets; cook until firm, about 3 minutes per side. Remove fillets from the skillet to a place and set aside.
Stir fish sauce, soy sauce, and oyster sauce together in the skillet. Add onion and red bell pepper; cook and stir until softened, about 4 minutes. Return catfish fillets to the skillet and spoon sauce and vegetables over the fillets. Continue cooking until the flesh of the fish flakes easily with a fork, about 3 minutes more. Garnish with green onions to serve.