Boil Pasta: Cook the orzo in a large pot of salted boiling water until al dente, about 9 minutes.
Cool Pasta: Drain the orzo in a strainer and rinse thoroughly with cold water to cool it down and remove starch. Transfer to a large bowl.
Mix Dressing: In a jar, combine the olive oil, reserved sun-dried tomato oil, red wine vinegar, garlic, basil, salt, and pepper. Shake until emulsified.
Assemble: Pour the dressing over the cooled orzo and toss. Gently fold in the chopped tomatoes, spinach, and red onion.
Finish: Sprinkle the crumbled feta cheese over the top and gently stir. Serve immediately or store in the refrigerator.