Is "Kollege" pronounced with an open O [kɔˈleːɡə] or a closed O [koˈleːɡə] ? I'd expect to pronounce that word with an open O because 2 consonants follow the O (but exceptions to this rule exist, eg hoch) and Wiktionary confirms it (https://en.m.wiktionary.org/wiki/Kollege). However, I think I hear a closed O in all Forvo audio files and in Google Translate.
It is only in stressed syllables that the opposition between /oː/ and /ɔ/ is distinctive and well-defined. In a unstressed syllables, it is not distinctive (and in reduced syllables, it cannot occur). Consequently, there will always be an uncertainty as to whether such an o should belong to the phoneme /oː/ or /ɔ/.
This means the exact pronunciation does not matter. You can pronounce the o as [o] or [ɔ] or something inbetween.
This is actually a rather interesting question. In all my life I (think I) have only ever heard [o], the closed O by native Germans. However, this goes against the ‘usual’ pronunciation rules of German according to which a short (‘unstressed’) O should always be open. It has already been mentioned that the Duden gives [kɔˈleːɡə] as a pronunciation. A very similar word and interesting candidate is Collage because it shares the /ko/ beginning and which the Duden gives as [kɔˈlaːʒə]—a pronunciation I agree with much more although testing it just now it seemed okay with both [o] and [ɔ].
The Wikipedia article on German phonology linked above already holds the clue to what is going on here: all these words are foreign loanwords and there are other foreign loanword examples where a short O is much closer to [o] than [ɔ] in (what I perceive as) standard pronunciation: Motór, Psychologe and kolorieren. In these cases, I believe that the closed [o] pronunciation is closer to the pronunciation in the original language.
Further obfuscation might have come from the fact that a number of other words beginning in unstressed Ko- derive from the Latin prefix co- which is usually pronounced [ko]—except where it is adjusted to kom- (Latin com-) or, more rarely, kol- (Latin col-) … Although ironically the Kol- in Kollege is exactly that: a co- adjusted to col- due to stem-initial L.
: The open [ɔ] pronunciation is one I have definitely heard before when I think and remember hard enough, but it has the distinct ring of being ‘Turkish-German’. In fact, trying to start pronouncing Kollege with [kɔ] makes me want to finish the word as Kollegas, a stereotypical expression that young men of Turkish origin would be expected to use towards their peer group.
: Motor can be pronounced with emphasis on either syllable. If the first is emphasised, both vowels end up being long and pronounced as [oː]. If the second is emphasised (I used the acute accent to show this), the first syllable becomes short but the vowel quality does not change and it most certainly does not become [ɔ].