I've searched all around the compendium and forums but can't seem to find any info about oak boards specific to UOR. What's their purpose?
They are used to make ladders and boats. I'm not sure if there is anything else but I know those things are part of the use.
In an upcoming patch you will see more use for oak boards as well. I will get them added to our new item listing in the compendium soon. http://www.uorenaissance.com/info/?page=info_itemlist&itype=Item
There are also some other craftables with oak, such as chest with 3 drawers, church pews, oak table with runner, oak counters, etc. to name a few. I think some of the new bed designs use oak too. The reason for the value, and to answer where you get them: You need 85+ Lumberjacking to have a chance (I believe, or close to there) to chop oak from a tree. The oak 'veins' are scattered throughout the forest in a pattern. Like mining any tree that you get oak from once, will always have a chance at producing oak. The reason for its value, is that even at GM LJ, its still only a small chance to get oak instead of regular logs. For example, when I am LJing just trees that can produce oak, I still end up with 2:1 regular to oak boards. Also even at GM you have a chance to fail to produce anything when chopping a tree that can yield oak logs.
Oak comes from trees, it functions similar to how mining can yield different ores. Once you have sufficient lumberjacking, you'll start to get it from various trees around the world. The trees don't look special and they spawn all over the world, but once you find oak on a tree, it will always spawn oak.