Cook the orzo according to package directions. Drain and rinse with cool water to stop the cooking. Let it sit in the colander while you finish the vegetables and the dressing.
Meanwhile, heat 2 tablespoons of the olive oil in a large skillet with a lid over medium-high heat. Add the onion, season with salt and pepper, and sauté for about 2 minutes until the onions just start to soften. Add the spinach and continue to saute until the spinach is just barely wilted, about 2 minutes.
While the spinach cooks, in a large bowl, combine the remaining 2 tablespoons of olive oil, vinegar, salt, and pepper. Add the cooled orzo and spinach and the feta, if using, and toss. Serve at room temperature.
Notes
This spinach orzo salad can stay covered in the fridge for at least 3 days. If you are planning ahead, you might add the feta just before serving so that it stays in perky little crumbles. This is a very portable salad, perfect for picnics and potlucks.