Prepare grill or heat broiler.
Combine vinegar, mustard, horseradish, garlic, oregano, salt and pepper in food processor. With machine running, slowly add oil. Remove 1/2 cup and add to steak in resealable plastic bag. Reserve remaining dressing. Marinate meat 20 minutes.
Grill or broil steak 5 inches from heat 7 to 10 minutes per side. After turning steak, sprinkle with 2 tablespoons of the feta cheese, spreading evenly over meat.
Meanwhile, cook orzo according to package directions. For the last 2 minutes of cooking, add spinach. Drain; transfer to serving bowl. Add remaining feta and olives. Drizzle with reserved dressing. Serve salad warm or chilled with sliced steak. Makes 6 servings.