So the roots, enough of them, anyway, of that Oak are still in the ground?
That's no going to make it any easier to cut it up and get all the roots... they're still growing... down, sideways, etc. Sounds like you really need a
professional to handle removing it. I'd suggest a
tree professional... most
professional accountants would only tell you how much it will cost, and
professional pilots would tell you to keep lots of altitude and you'll never see it and
professional web designers will just try to use Flash on a web site about fallen trees... yep, stick with the tree kind... You're welcome.