For Oxford spelling the rule is simple:
Prefer the spelling the Oxford dictionary lists first for a given word.
Roughly speaking, Oxford spelling is British spelling with -ize instead of -ise. In computer circles it is called en-GB-oed. On Apple machines and the Chrome Web browser, Canadian English comes the closest.
Who uses Oxford spelling:
- Oxford English Dictionary (OED) – by definition
- Oxford University Press
- United Nations, including UNESCO, WHO and so on
- Amnesty International
- Encyclopaedia Britannica
- this blog
How Oxford spelling compares: I chose 14 Oxford spellings from the “Concise Oxford English Dictionary” that are not universal and used Google to compare them to the preferred spelling in 15 countries and 5 news websites:
- Oxford spellings: ageing, aluminium, analyse, centre, fetus, fulfil, haemorrhage, honour, judgement, practising, recognize, spelled, travelling, yogurt.
- With 3 differences:
- Canada (Chrome spell checker): fulfill, hemorrhage, yoghurt (? – both yogurt and yoghurt are counted as misspellings).
- With 4 differences:
- Al Jazeera: foetus, judgment, practicing, recognise.
- Times of India: foetus, judgment, practicing, spelt.
- South Africa: recognise, spelt, practicing, yoghurt.
- Britain, Australia, New Zealand: foetus, recognise, spelt, yoghurt.
- Ireland: foetus, judgment, recognise, yoghurt.
- With 5 differences:
- BBC: foetus, practicing, recognise, spelt, yoghurt.
- The Economist: aluminum, judgment, practicing, recognise, yoghurt.
- Jamaica: aging, fulfill, judgment, practicing, spelt.
- With 6 differences:
- Nigeria: analyze, fulfill, judgment, practicing, spelt, yoghurt.
- Canada: aging, aluminum, fulfill, hemorrhage, judgment, practicing.
- Singapore: aging, fulfill, foetus, judgment, recognise, spelt.
- With 7 differences:
- India, worldwide: aging, analyze, foetus, fulfill, judgment, practicing, spelt.
- Pakistan: aging, aluminum, analyze, fulfill, hemorrhage, judgment, practicing.
- Uganda: aging, foetus, judgment, practicing, recognise, spelt, yoghurt.
- With 8 differences:
- Philippines: aging, analyze, center, fulfill, hemorrhage, honor, practicing, traveling.
- With 9 differences:
- With 10 differences:
- America, New York Times, Israel: aging, aluminum, analyze, center, fulfill, hemorrhage, honor, judgment, practicing, traveling.
Notice that most follow neither a pure British nor a pure American spelling.
(By the way, the BBC and the New York Times are the farthest apart: they have only one disputed spelling in common: practicing.)
The main difference between the three spellings concern -our and -ize: recognize and honour. Oxford sides with most of the English-speaking world on those:
honor honour recognise Singapore, The Economist,
Ireland, BBC, Britain, New Zealand, Australia,
Al Jazeera, South Africa, Uganda
recognize America, New York Times, Israel, Philippines OED, this blog, worldwide,
Jamaica, Nigeria, Canada,
Pakistan, India, Times of India
On some other spellings, though, Oxford is outnumbered:
judgment, fulfill, analyze, aging, spelt, foetus, practicing
Which are the Indian spellings. India, it turns out, is the swing vote.
The OED prefers spellings that are closer to a word’s origins: fetus is closer to the original Latin, -ize and analyse to the original Greek and so on.
I follow the Oxford spelling because I use the Oxford dictionary (since it is the best there is). I used to follow The Economist’s spelling, but it is much easier to look up the Oxford spelling for a given word. Also, as even the United Nations recognizes, OED English is the closest thing we have to a world English.