In a 5- to 6-quart pan over high heat, bring about 3 quarts water to a boil. Add orzo and cook, stirring occasionally, until tender to bite, 7 to 12 minutes. Drain, reserving 1 1/2 cups pasta-cooking water. Return orzo to pan.
Meanwhile, heat oil in a 10- to 12-inch frying pan over medium heat. Add garlic; stir until fragrant but not brown, 1 to 2 minutes. Add chili flakes and broccoli rabe; stir 3 to 4 minutes. Add 1 cup of reserved water; simmer until greens are tender to bite, 6 to 8 minutes.
Add broccoli rabe mixture to orzo with ricotta, olives, and remaining 1/2 cup reserved water; mix, adding salt and pepper to taste.