Oak is an attractive hardwood that's used for a variety of things -- from boats to decks to floors. However, oak is just as vulnerable to water and the damage it can cause as other woods. But, if you take precautions and you put a barrier between the oak and the water, then you can, in a sense, make your oak water proof. All it takes is time and the right materials, like polyurethane paint and a paintbrush.
Clean the oak off so there's no dirt or grime. Sand down or cut off any imperfections in your oak. This will remove flaws from the oak that you don't want seen, but it will also create a rougher texture that will hold harder to the polyurethane coating that you put on the oak.
Paint on a coating of polyurethane paint. You can get this paint in a variety of colors, or in a clear coat so that you can see the oak beneath. Paint on a single, smooth coat and cover all of the areas you want protected. This may require painting half of the oak, and then letting it dry before you turn it over and paint the other side.
Paint on an additional layer of polyurethane paint once the first coat is dry. You can put on a third coat once the second has dried, but two coats should be enough to protect your oak from water and to keep it dry and insulated.
If a crack develops in the paint, due to an impact or just due to time, you'll need to repaint the oak to maintain protection.