Meanwhile, heat the olive oil over medium heat in a large skillet or shallow saucepan. Add onions and cook for 5 minutes to soften, stirring to prevent browning. Add the garlic, diced tomatoes, and oregano. Cook for about 10 minutes and add the chopped spinach. Cook for another 5-10 minutes until the spinach has softened and the flavors developed. Add salt & pepper to taste.
When the sauce is nearly done (about the time you add the spinach), cook the linguine according to the package directions.
Drain the pasta and place servings on individual plates. Spoon sauce on top. Sprinkle with feta and serve immediately.