Heat 2 teaspoons vegetable oil in a skillet over medium heat; cook and stir mushrooms until the juice has evaporated, and the mushrooms are browned, about 15 minutes. Set mushrooms aside.
Fluff cooked rice with a fork and stir in 1 tablespoon vegetable oil to separate the grains. Heat remaining 3 tablespoons vegetable oil in a large skillet or wok over medium heat. Cook and stir pork, garlic, and ginger until fragrant, about 1 minute; add celery, onion, and carrot. Cook and stir until vegetables begin to soften, 3 to 4 minutes. Stir in red bell pepper and cooked mushrooms just until combined.
Stir in cooked rice, lightly tossing until rice is heated through and thoroughly combined with pork and vegetables. Stir in soy sauce, remove from heat, and sprinkle green onions over the top.
