In a small bowl, mix together the oregano, paprika, cumin, and salt.
Pat the chicken thighs dry, then rub them all over with the spice mixture.
Add the olive oil to a large, high-sided pan and turn the heat to medium-high. Fry the chicken for a few minutes on each side, until golden brown. Remove the chicken from the pan and set aside, keeping the juices from the chicken in the pan.
Add the onion and pepper, and fry them until softened — about 8-10 minutes. Add the garlic, and cook for another minute.
Add the rice, and cook for 2-3 minutes, until lightly toasted.
Add the chicken stock, chopped tomatoes, olives, and olive juice. Stir well to combine, and bring to a boil.
Place the chicken back into the pan, cover, and reduce to a simmer. Leave to cook for 30 minutes.
After 30 minutes, the liquid should have absorbed and the rice should be cooked. Season to taste, and serve with fresh chopped parsley, if desired.