Preheat oven to 425 degrees F.
Place chicken thighs and legs on a sheet pan. Lightly coat with oil and roast until golden, about 35 to 45 minutes.
Coat a large, deep stock pot lightly with olive oil. Add the onions, celery, carrots and garlic to the pot and bring it to a medium high heat. Cook the vegetables stirring frequently until they start to get soft and are very aromatic, about 8 to 10 minutes.
Remove the chicken from the oven and add to the pot of vegetables along with the bay leaves and thyme. Fill the pot with water. Place the pot on a burner on high heat. Bring to a boil and reduce to a simmer. Eventually a gray scum will form on the top of the water. Skim the scum and the fat off the top of the water. Simmer for 2 1/2 to 3 hours. Refill the water as it evaporates.
Strain stock and discard the chicken and veggies. Store the stock in smallish containers, plastic pint and quart containers from Chinese take out are perfect! Recycle! If not using right away freeze for later use.