Place the dried shiitakes in a small bowl and cover with very hot water. Soak about 15 minutes, until tender. Drain and reserve liquid. Discard the mushrooms' stems, and chop.
In a small skillet over medium-high heat, sauté the fresh shiitakes in 2 tablespoons of the peanut oil. Cook, stirring occasionally, until the mushrooms turn golden-brown and crisp. Transfer to a small dish.
Heat the last 2 tablespoons of the peanut oil in a wok or large skillet over high heat. Add the bok choy stems, garlic, re-hydrated mushrooms, and 1/4 cup of the reserved mushroom water. Cook 3-4 minutes, stirring frequently, until the bok choy is crisp-tender. Fold in the bok choy leaves and stir in the oyster sauce. Cook 1-2 minutes, until the greens wilt. Serve immediately, topped with the reserved crispy mushrooms.