Heat olive oil in a large skillet over medium-high heat.
Season chicken thighs with salt, pepper, and half of the oregano.
Add the seasoned chicken to the skillet and cook until browned, about 5 minutes per side.
Remove the chicken from the skillet and set aside.
In the same skillet, add the sliced onion and bell pepper; sauté until the vegetables are softened, about 5 minutes.
Stir in the minced garlic and remaining oregano, cooking for an additional minute until fragrant.
Add the diced tomatoes and olives to the skillet, stirring to combine with the vegetables.
Return the chicken to the skillet and bring to a simmer, cooking for 10-15 minutes until chicken is cooked through and sauce is thickened.
Remove from heat and garnish with fresh parsley before serving.