Cook the penne in boiling salted water until al dente; according to package instructions. Drain, rinse, and set aside.
While the penne cooks, make the vinaigrette. In a small bowl, whisk together the olive oil, vinegar, garlic, oregano, and kosher salt; set aside.
In a large bowl, combine the cooked penne, spinach, tomatoes, red onion, and feta. Pour on the vinaigrette and toss until well combined.
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Notes
Use baby spinach if you can find it. Baby spinach are just spinach leaves that are harvested earlier and are more tender with a slightly sweeter flavor.
Cherry tomatoes work great here because they don't get as wet and mushy as diced tomatoes.
Penne is a great shape for absorbing the flavors in this pasta salad. You can use any short pasta shape you like.
This salad can be made ahead and refrigerated for up to three days. If making ahead, add the feta crumbles right before serving.