It might be unique to the OED, but they do have historical evidence to back up their case.
I understand etymology has to use history to present a case, but I don’t know a single BrE speaking person that’d write a current text with -ize endings. The OED aren’t prescribing current usage with their stance.
As a BrE speaker, I do use "ise", but "ize" is not unknown, especially in science (I believe that the British published journal Nature has it as their house style). Recently I think it has been declining as it has been identified as "the american version".
In mainstream usage that's true, but scientific usage leans more to the OED spelling, especially since some major journals (even UK-based ones like Nature) enforce it.