Brown oak is oak naturally coloured by the bracket fungus Fistulina hepatica, commonly called the beefsteak fungus due to its blood red colour.  Often mature trees with a branch which has fallen off will attract the fungus and the colouring starts at the ‘wound’ and slowly penetrates the tree,  so there may be a mix of white and brown in some trees,  running to solid brown eventually.  Brown oak is rare to discover and valuable as such.  In some areas it can be less durable, hard and strong but generally retains quite adequate strength properties as timber.