It may be very difficult to remove this. Wood and water are generally not good companions. On top of that oak is a very porous wood and the water and accompanying discoloration that it leaves behind may very well have penetrated down into the wood more than can be removed by a light sanding.
Note that staining like this very often happens at the boundary of how far the water got and the last place for it to dry.
Some recommendations for removing a water stain on unfinished wood consist of making a paste of salt (only 1 or 2 drops of water per teaspoon of salt). Apply the paste to the water stain and gently rub the paste into the water stain with a damp towel. After that wipe away the paste and look if the stain has been removed. It may be necessary to try this process several times.
There are other techniques that people have tried including water staining the whole surface to even out the marks, salt cleaning followed by a hot iron treatment, bleach, lemon juice......but you would have to take most of those with a grain of salt. (pun intended) You would just have to try and see if anything works.