I wouldn't worry, but if it will make you feel better, the best protection would be to add a dynamic cabling system in the crown, 2/3's of the way up the tree. One of our site sponsors, Sherrill, sells Cobra and a few others too.
In time, it may begin to rot and perhaps even rot enough to drain the water. The tree will then work to repair the damage and maybe close it up and refill the pond. This could cycle like that for 100 years, who knows?
The tree looks like it was once cut down and three or more stems grew up from the stump. Now their bases are growing together. Although as you surmised, this base configuration is not the strongest, because the root flare doesn't grow well in the center. Off setting this weakness is the protection each stem gives the others from wind loads, and the fact that Oak is strong, rot resistant wood. Also, when a tree has a weakness that allows movement, this movement signals the tree to grow stronger in that area of movement. We often see trees that have a damaged buttress root, have a larger than normal buttress root configuration on the rest of the tree.
This is why I recommend a dynamic cable, that allows free movement of the stems, and then only holds if the tree moves beyond a certain point. This movement helps the tree to continue to grow stronger in the weak area because of that movement.