Zest the lemon into a small bowl and set aside. Squeeze the lemon juice (at least 1/4 cup) into a separate, larger bowl.
Prepare the dressing: To the lemon juice, add olive oil, oregano, salt, and pepper. Mince two cloves of garlic and add them in, along with a pinch of lemon zest. Whisk until well combined.
Marinate the chicken: Place chicken thighs in a zip-top bag. Add half of the dressing. Mince the remaining garlic clove and add it to the bag. Seal and massage to coat. Refrigerate for at least 30 minutes.
While chicken marinates, cook pasta according to package directions. Drain, rinse with cold water, and set aside to cool.
Dice the cucumber and bell pepper, halve the tomatoes, slice the olives, and chop the parsley.
Heat a skillet over medium heat. Cook the marinated chicken thighs for 5-7 minutes per side, until browned and cooked through. Let rest for 5 minutes before chopping into bite-sized pieces.
Assemble the salad: In a large bowl, combine the cooled pasta, cucumber, bell pepper, tomatoes, olives, parsley, and chopped chicken.
Crumble the feta over the top, add another pinch of lemon zest, and pour the remaining dressing over the salad. Toss to coat and serve immediately or chilled.