A cord of ash has 21.6 MBtu available in it so 6 cords =129,600,000 BTU in it. If you're burning it in a 60% efficient stove that makes 77,760,000 Btu of heat.
There's 140,000 BTU in a gal of fuel oil, so 600 gal = 84,000,000 BTU. If the oil furnace is 79% efficient, that's 66,360,000 BTU...