In a large pot, bring water to boil for pasta. Cook till al dente, drain and set aside.
While pasta is cooking, in a large skillet heat olive oil over medium high heat.
Add garlic and onion, and saute till translucent, but not brown.
Add chicken and cook until chicken is done, about 5 minutes.
Reduce heat to medium low and add tomato, feta, lemon juice and oregano.
Add pasta. Stir until completely heated through, salt and pepper to taste and serve.