Whilst this heavy reduction is more than the recommended 30% and subsequently will look a little severe, it is a preferable alternative to removal.
An oak tree can be expected to live for up to a thousand years and provides habitat for over 300 species of invertebrate in addition to providing valuable food sources for a wide variety of mammals and birds, most famously the Eurasian Jay.
Allowing an Oak tree to remain and die of its own accord is of the utmost ecological importance.