Heat olive oil in a large skillet or Dutch oven over medium-high heat.
Season chicken cubes with salt, pepper, and oregano, then add to the skillet. Sear until browned on all sides, approximately 5-7 minutes. Remove chicken and set aside.
Add minced garlic and dry orzo to the remaining oil in the skillet. Toast the orzo for 2 minutes, stirring constantly until fragrant and lightly golden.
Pour in the chicken broth, scraping the bottom of the pan to deglaze. Bring to a boil, then reduce heat to low.
Cover and simmer for 8-10 minutes, or until the liquid is mostly absorbed and the orzo is tender.
Stir in the seared chicken, baby spinach, sun-dried tomatoes, and olives. Heat for 2-3 minutes until the spinach has wilted and the chicken is cooked through to an internal temperature of 165°F.
Remove from heat. Stir in the lemon juice and top with crumbled feta cheese before serving.