Configuring a Mac to use Oxford spelling
LibreOffice's British English dictionary has support for Oxford English. You can download the .dic and .aff files from https://github.com/ropensci/hunspell/tree/master/inst/dict and place the files in ~/Library/Spelling – either manually, or using the terminal:

wget https://raw.githubusercontent.com/ropensci/hunspell/master/inst/dict/en_GB.aff -O ~/Library/Spelling/en_GB.aff
wget https://raw.githubusercontent.com/ropensci/hunspell/master/inst/dict/en_GB.dic -O ~/Library/Spelling/en_GB.dic


Then log out and log in again, or restart your machine.

You should now be able to go to your keyboard preferences (Apple menu → System Preferences…), choose the Text tab and enable the new dictionary under Spelling. It should be called “British English (Library)”:



Here's a screenshot of a Safari text field spell checked with this dictionary:



P.S: If you choose to enable auto-detection of the language used for spell checking, it doesn't seem to use these “Library” dictionaries, so you might need to right click in text fields, open Spelling and grammar and explicitly set the language to British English (Library) to get it to work.