Cut chicken into pieces and place in a bowl. Add olive oil, oregano, zest and juice from lemon, and salt and pepper to taste. Cover and refrigerate for at least an hour (can make ahead and refrigerate overnight).
When ready, heat a large, non-stick skillet over medium-high heat. Add chicken and sauté until cooked through, about 5 to 8 minutes. Remove from heat and set aside.
Prepare orzo by bringing broth/water to a boil in a large saucepan. Stir in orzo, reduce heat to medium, cover partially and simmer, occasionally stirring, until al dente. Drain in a colander and rinse briefly in cold water to cool down and rinse off excess starch, being mindful not to rinse off flavour from chicken broth! Transfer to a large, wide bowl.
Mix tomatoes, feta, basil, green onions and cooked chicken into orzo. Add dressing; toss to coat. Serve at room temperature (leftovers can be served cool.)