Bring a quart of salted water to a boil in a saucepan. Cook broccoli rabe until tender crisp, about 3 or 4 minutes; drain. Wipe pot dry, add oil and heat over medium high. Add garlic and fennel and cook, while stirring, until fennel is softened, about 5 minutes. Stir in the broccoli rabe, oregano, and salt. Cook until greens are heated through, about 2 or 3 minutes.
Transfer to a serving dish and toss with the feta cheese.