spelling standard used by the Oxford University Press (OUP) for British publications, including its Oxford English Dictionary (OED)
Oxford spelling is the spelling found in the Oxford English Dictionary. One example would be "colour" instead of "color".
More exactly, Oxford spelling is the headwords used in their dictionaries. In the full entries other variations are listed.
Most people in Britain use spellings like "organise" and "realise", but Oxford spelling uses "organize" and "realize". Those alternatives are both correct in British English.