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 the crock of 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 bouillon in the boiling water, and 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, and cook on Low setting for 8-10 hours, or on High setting 4-6 hours. A meat thermometer, inserted into the thickest part of a thigh, not touching a bone, should read at least 160 degrees F (70 degrees C).