Willow oaks are somewhat fast at creating callous tissue, however, I share BCMA's opinion.
I agree too, the problem is that no tree can close such a large wound fast enough to fully compartmentalize. The wound will probably not be close enough to any green mass to support fast growth either.
Another problem is that a this limb to stem ratio tells me that the limb probably originated from a lateral bud subordinate to the terminal as a young tree. The branch/stem pith then conjoin so the decay court has a higher probability of filling the entire tree.
Gillman calls it codominance, physiologically this is correct, due to the branch and stem originating from the same node, but IMO it confuses because it uses a term for a different structural characteristic.
I prefer to think of it as primary and secondary branching. Primary is where the buds originate from the same node and pith conjoins, and secondary is the more common, where the branch came from a younger bud.
The biggest reason for the differentiation is that there is no collar with primary branching, or branch protection zone
http://www.treelink.org/joa/2002/march/06Eisner.pdf
Branches constitute a point of weakness to the health of trees, as the primary points of infection for trunk decay(Toole 1961; Shigo 1975). The importance of the branch protection zone (BPZ) in limiting trunk infection after branch injury or removal has been demonstrated
(Aufsess 1975, 1984; Shigo 1985). BPZs are generally
recognized as cone-shaped areas of discolored wood at
the base of dead branches. The xylem within these BPZs
is occluded with decay-resistant compounds
Newer definitions of the BPZ include the physiology of the connections of xylem between the stem and branch, there is a fold, or truncation that limits fluid transfer. So the BPZ is a woody structure and the discoloration is a reaction zone formed in it. Since the primary branching lacks this structure, the reaction zone is weaker.
If you cut back to a node now, sprouts will start to grow in the spring. Next fall, you should be able to thin back to the two or so most viable sprouts with the best attachment. From then on, it will take a little bit of maintenance to reform a nice scaffold limb (reduce length of sprouts, remove redundancies, etc.), but in 5 or so years, there should be a sizable limb with good structure there. The most important part is making sure you cut back to a node, and then only keep the sprouts with the best attachments.
I would wait two years, maybe three before beginning the restoration thinning, the more green there is, the less energy the tree needs to pull from it's sinks to make the reaction zones. Reaction zones are chemical changes, which require energy use.
Gillman has shown that there is a direct relationship with effectiveness of compartmentalization and proximity to green (dynamic) mass.
