Season the chicken breast with salt, pepper and half the oregano. Heat a grill pan over medium-high heat with a little olive oil and grill the chicken for about six minutes per side until cooked through. Let the meat rest for a few minutes under a piece of foil.
Meanwhile, cut the tomato into chunks, the cucumber into half-moons and the red onion into thin rings. Place everything in a large bowl and mix with the olives.
Whisk together the olive oil with the red wine vinegar, the remaining oregano, salt and pepper to make a dressing and toss with the salad. Slice the chicken into strips, arrange on top and crumble the feta over it.