In a large pot of boiling salted water, cook pasta according to package instructions, adding the broccoli rabe 4 minutes before the end of the cooking. Meanwhile, in a small saucepan, heat the oil, garlic, and red-pepper flakes over medium until the garlic begins to sizzle, about 2 minutes. Drain pasta and the broccoli rabe and return it to the pot. Add oil mixture and toss to coat; season to taste with salt and pepper. To serve, sprinkle oregano over pasta and drizzle with lemon juice.