The stress of this word is "ciAtion" part. Apart from the stress, let's look at first two syllables.
Here, you clearly see schwa ə sounds, instead of "o", "ou" or something else.
Therefore, vowls of "pronun" part may sound very similar. In practice, it can soud different because speaker's tongue and lip prepares for a coming consonant. Thefore, there are (probably) some variations among speakers accent how it sounds.
We should, however, keep in mind that the IPA of "pre nun" part is the same.
When it comes to long syllable words, this happens frequently. Here are some examples:
There are some exeptions. But you see that there are many letters which are reduced to schwa sounds.