How do I keep an outside oil line from freezing? It has insulation on it already.

Buy and wrap electric heat tape like you would use on water pipes around the plumbing. For the actual tanks you could use an electric blanket. There are some electric blankets that are actually intrinsically safe (safe to use in gas-laden environments), but as long as there are no strong vapors from the tanks, then a regular electrical blanket would be okay. Intrinsically safe blankets are very expensive, and if it is so cold that you need to use a blanket to keep the tanks warm, then there really should be no vapors unless you really turn the heat up on the blanket. Another option would be to look into fuel oil additives for the cold, like diesel fuel additives.