My Oxford dictionary says 'organization' is pronounced 'orga-NIZE-ation' in British English and 'orga-'NIZZ'-ation' in American.
Is that correct or do both forms exist in both varieties of English?

 

Your Oxford is partly correct in that Americans often say it as orga-NIZE-ation, too.

 

I agree with MuttQuad.

You can listen to British and American pronunciations of organization here.
I think I unconsciously alternate between the two.

 

You will hear both in Ireland. I wouldn't classify either as being primarily British or American.

 

Hello!
In this word, the stress is on the "a" of -"ation", so the vowel that we're talking about comes directly before a stressed syllable. Vowels in this position are commonly reduced in many varieties of English. As a speaker of American English, I would most likely not pronounce this with either a clear "ize" or "izz" vowel, but with a schwa. So /ɔrgənəˈzeɪʃən/. However, the closely related word "organize" has a different stress pattern and a clearly pronounced 'ize": /ˈɔrgənaɪz/ so, if I was for some reason emphasizing all of the syllables of the word "organization" or saying it syllable by syllable, I would definitely use the "ize" vowel and not the "izz" vowel. In fact, I somewhat doubt that any speakers have an "izz" vowel rather than just a schwa in this word; my pronunciations of an unstressed /ɪ/ and a schwa are very similar, and it's hard to tell these sounds apart just by listening.

 

As an AmE speaker, your unstressed i and schwa are likely to be similar. However, BrE sepakers will often leave some i in the unstressed situation. resulting in a "schwi" - this being the likely version you describe as "izz". Singers of the US national anthem usually use per-uh-luhs for perilous, while a BrE seaker would have some i left in that word)

 

How I would look at it is that there is a process available to English speakers of vowel reduction in certain phonological environments, such as in the syllable immediately preceding the stressed syllable. Certainly, there are differences between speakers in how much and how often they reduce pre-tonic vowels. In fact, I hear a diphthong in the American’s pronunciation in the link provided by aloofsocialite, so we can see that this is not a simple matter of “British” vs. “American” varieties (few things truly are).
I am arguing that /aɪ/ is the only unreduced pronunciation, and there are also reduced forms, with the level of reduction depending on the speaker. This would mean that apparent instances of the sound [ɪ] would in fact be the realization of this reduced vowel. It may seem academic to distinguish between the /ɪ/ that shows up in stressed syllables and the reduced "schwi" vowel that we see here. However, one prediction of the model I’m presenting is that the [ɪ] sound would only pop up in this word in contexts where vowel reduction is possible. That is, I would predict that speakers like Angela Rippon, if asked to say each syllable one-by-one, would produce ɔ.gan.aɪ.zeɪ.ʃʌn or perhaps ɔ.gan.aɪz.eɪ.ʃʌn rather than ɔ.gan.ɪz.eɪ.ʃʌn. But I don’t actually know-- perhaps some speakers do in fact have the [ɪ] sound in all contexts for this word.

[Note: another reason it can be useful to distinguish the reduced vowel "schwi" from a "short i" is that schwi can be represented by other letters than i, such as the e in "equator".]

 

I think if you asked me to say each syllable separately, I'd use ai for the i in ization (true for any word suffixed with that) but often use the schwi when saying the word at normal speed. I think I also use both the schwi and the short ai at random when saying the whole word.

 

Thank you very much for your in-depth replies.