Hi, 

These look like they are the begining forms of Rough Oak Bullet Galls. There are many different types if galls that Oak trees get so it is possibly a different kind but regardless, they are not harmful to the tree. 

Galls are usually formed from a wasp, midge or mite, that lay eggs in the tissue of the wood and cause the cells to grow in a different form. The larva then shelters in the gall protected and with a food source until time to emerge. Sometimes predatory insects will feed on the larva so you may see holes in the galls, or it could be from the larva exiting the gall. 

No need to be concerned, just a cool part of nature! The links provided explain this in detail with lots of good photos! 

Emily