Mix the ingredients for the marinade except for the vegetable oil, and toss with the shrimp in a bowl. Marinate the shrimp for about one minute, stirring once. Then add the 1/2 tsp. vegetable oil and set aside.
Heat the half-cup of vegetable oil in the wok over medium heat. When the surface of the oil starts to ripple, strain the shrimp out of the marinade (reserve marinade), and add them to the hot oil. Fry until the shrimp turn pink--about a minute--then add the mushrooms. Continue to cook for about 15 seconds. With a skimmer, remove the shrimp and mushrooms from the wok and set aside.
Pour off the oil from the wok and wipe. Heat the wok over high heat and return the shrimp and mushrooms. Stir in the garlic, green onions, oyster sauce, soy sauce, and reserved marinade. Stir-fry shrimp until browned in spots. Sprinkle with a little crushed red pepper and serve. (Good with steamed broccoli flavored with some of the sauce.)