I recorded these samples after a discussion in Usenet group nl.taal, where there was doubt as to whether a "pure", fully monophthongal [o:] sound could be used in a Dutch word such as "lopen", without sounding unnatural. I think it can be used, and is used, in Eastern parts of the Netherlands, including areas where forms of Low-Saxon are spoken (with the exception of Groningen, where the o-sound is diphthongal).
Another interesting phonetic phenomenon is that the strongly diphthongal realisation of the Dutch long o-sound heard in "plat Rotterdams" (Rotterdam city dialect) is so much similar to the sound used in British "Received Pronunciation". This was discussed on the Lowlands-L mail list in 1997.
The sounds are very much alike, but not identical, or perhaps
I should say, not all variants of the sound are found equally
often in both language varieties.
By recording these sounds too, I have attempted to illustrate the subtle difference, which I think is that in Rotterdam, the diphthongs starts central unrounded, but ends much more rounded and back, whereas the English sound starts about the same, but also ends somewhat central and not very rounded.
To make comparison easier, I have inserted the English sound into the Dutch word, although it is not an English word, and in the case of the monophthongal o, it is not supposed to be a Low-Saxon word (I'm not even sure if it exists in Low-Saxon in that form), but rather a Dutch word as pronounced by a Low-Saxon speaker when speaking Dutch.
There is now also an imitated Australian o, which I think goes very much into the direction of a central rounded sound, or nearly even a front rounded sound, the [y] of French "lune".
A note regarding the authenticity of these pronunciation examples:
I can only claim native speaker status, even if with hesitation,
in the case of Rotterdam, considering the fact that I was born
there, and lived there and in a neighbouring town until the age of 12.
Yet, more than 30 years have passed since I left that part of the country.
The other three recordings are no more than my imitations of what I believe to have heard from others, so they may be inaccurate.
By the way, the Australian sound in "no" is very similar to what speakers in Northern-Ireland use in the word "now". Just a coincidence, I suppose.
Yet another strange coincidence:
The Rotterdam sound, ([3u] in X-SAMPA notation) when nasalised, is practically identical with the Portuguese sound written ão (sometimes am). Both go from half-open central unrounded to close back rounded.
Even stranger is that although I am familiar with the Rotterdam sound from the late 1950s, and with the Portuguese sound from the mid 1970s, I only noticed the similarity in November 2000.
See also this note.