ok.. I would just like to clarify something - I do in fact make my students aware that zero is commonly pronounced as the letter 'O'. I teach to German people, most of whom are already aware of this due to years of English courses in school, etc..
In German, the word for 'zero' is 'Null' and it's always used for phone numbers. SO, 'Null'='zero'. That is the unamiguous, technically correct and exact translation. I teach the students that it is more correct to say 'zero' because it is literally not the letter 'O', no matter how post-modern you wish to be. I don't make a fuss over it, I'm fully aware that many/most people use 'Oh', and I don't care if my students use 'Oh'. I even use it sometimes myself. It's just not a big deal. As I stated in a previous post, I just tell the students 'the full story'. They are never confused about it afterwards and very little time is required to be spent on it. Similarly, I've spent enough time on it here as well. The End.
In regards to the related topic - whether or not there is actually such a thing as 'correct' English, my response is simply that in practice, 'on the ground' so to speak, an 'anything goes' approach is inappropriate. In a philosophy class it might be fine to discuss how everything's relative and so on, but in real world ESL teaching it's advisable to 'stick to the rules' of English grammar as much as possible. Of course language changes over time. Of course there's no perfect, absolute, master version of English set in stone forever. There are very cohesive standards and rules however(which change gradually over time) which make it possible for us to communicate clearly with each other, and to pass on the knowledge of our language to non-native speakers.
Take for example this (now online) book written 90 years ago, entitled 'How to Speak and Write Correctly':
<a href="http://www.gutenberg.org/dirs/etext04/hwswc10h.htm" target="_blank" title="http://www.gutenberg.org/dirs/etext04/hwswc10h.htm">http://www.gutenberg.org/dirs/etext04/hwswc10h.htm</a>
While the author's writing style may seem slightly antiquated, it's still fine and 99% of his description of English grammar still applies today, nearly 100 years later. The other 1% consists of the sort of changes and developents that are often discussed on this forum.
The problem of what is correct or not of course often comes up in TESL when British and American English use different words or spelling. At the end of the day you have to teach either one or the other, or teach both. Usually it depends on which one the client/student is most likely to need.