1. Preheat oven to 350°. Put bones and vegetables in a roasting pan and toss with oil. Roast until browned, about 30 minutes.
2. Spoon ingredients into a large pot and add 4 qts. water. Bring to a boil and spoon off any foam that forms on top. Reduce heat and simmer until broth is reduced by half, 2 1/2 to 3 hours.
3. Pour mixture through a strainer into a bowl and let broth cool; discard what is in the strainer.
4. Use broth, or ladle it into quart-size freezer bags or containers (great for soup) and/or ice cube trays (handy for sauces), and freeze for up to 6 months; transfer the cubes to bags once they're solid.