Rinse the chicken inside and out. Place one half of the lemon and 3 garlic cloves in cavity of chicken. Reserve other half of lemon.
Layer the sliced potatoes and the onions in a slow cooker. Sprinkle remaining 3 garlic cloves around the inside of the cooker; place chicken onto the vegetables. Pour in the wine and olive oil. Dissolve stock in the boiling water then add to the cooker.
Squeeze the juice of remaining lemon half over chicken (strain out seeds); sprinkle with oregano. Season chicken with salt and black pepper.
Cover then cook on Low setting for 8-10 hours (or on High setting 4-6 hours). Chicken is done when juices run clear.