Most often お is pronounced [ō], however sometimes I hear an [ä] instead. It might be arbitrary, but it sounds so consistent to me, thus I ask, are there? And if so, what are they?
Japanese is extremely constant which makes it such an easy language to learn. Every sounds are pretty much fixed and never change.
There are some tiny exceptions like wo -> o, ha -> wa, he -> e and sometimes the ん followed by a vowel makes a weird sound, but never heard of a different pronunciation for お.
This stability in the sounds in Japanese is much appreciated, unlike the excruciating randomness of languages like English(like how "read" is pronounced completely differently in the past tense even though it is written the exact same way.)
I think this is a common misunderstanding. I too thought that there were two pronunciations of お, so I was very confused when people not only told me it was ONE sound but also gave me negative feedback when I asked.
This was hard for me to understand, but even tho お in words like 青(あお) practically has another sound, that is only because it is outward to say it as written. If you listen VERY closely to a native speaker saying 青い, you will hear a faint "oh" sound even tho it's not as clear as you would want to, it's just a bit lazy sometimes.
This is a huge trap for beginners in Japanese languish, so I think it's a shame that there is no info about this online.
I hope this clears up some things.