In a medium bowl, whisk together the olive oil, lemon juice, minced garlic, dried oregano, salt, and black pepper to create the marinade.
Add the cubed chicken to the marinade, tossing to coat thoroughly; let sit for at least 15 minutes or up to 2 hours in the refrigerator.
Heat a large non-stick skillet or grill pan over medium-high heat.
Cook the chicken for 8 to 10 minutes, stirring occasionally, until the internal temperature reaches 165 degrees Fahrenheit (74 degrees Celsius).
In a small mixing bowl, combine the diced cucumber, halved cherry tomatoes, and red onion.
Divide the cooked grains evenly among four bowls.
Top each grain base with the cooked chicken, the cucumber-tomato-onion mixture, olives, and crumbled feta.
Garnish each bowl with a dollop of tzatziki sauce immediately before serving.