Spay a slow cooker with cooking spray. Add the chicken to the slow cooker. In a small bowl combine onion, garlic, oregano, salt, pepper, olive oil, red wine vinegar, lemon juice, and water. Pour over chicken. Cook on low for 6-8 hours or high for 4-6.
To make the sauce: In a medium bowl combine greek yogurt, cucumber, garlic, white wine vinegar, dried dill week, dried oregano. Salt and pepper to taste and add lemon juice and drizzle olive oil on top. Refrigerate for 30 minutes to let the flavors blend.
Prepare chicken on warm greek pita bread with desired veggies and sauce on top.