Slice the chicken breast into strips; set aside. In a large bowl, combine 2 Tbs. each olive oil and lemon juice, the oregano, garlic, salt and pepper. Add the chicken strips and toss to coat; cover and refrigerate 20 minutes.
Meanwhile, bring a large pot of salted water to a boil. Cook the orzo according to package directions, then rinse under cold water and drain.
Heat a large skillet over medium-high and add the chicken and marinade. Cook, stirring occasionally, until the chicken is cooked through, about 4 minutes.
Toss the orzo in a large bowl with the remaining 1 Tbs. each olive oil and lemon juice, the feta, tomatoes and baby spinach. Season with additional salt and pepper to taste. Top with the chicken and olives if using.