Preheat the grill to medium-high heat.
In a bowl, mix olive oil, oregano, salt, and pepper.
Coat the chicken breasts with the marinade.
Place the chicken on the grill and cook for about 6-7 minutes on each side.
Remove chicken from the grill and let it rest for 5 minutes.
While the chicken rests, prepare the vegetables.
Slice the cucumber, bell pepper, and red onion.
Halve the cherry tomatoes and set aside.
Cut the grilled chicken into strips.
Assemble the bowls by layering vegetables, chicken, and feta cheese.
Top with tzatziki sauce and enjoy!