In Israelic Hebrew its definitely /a/ sound. The bottom frontal one.
ָ = ַַ = ֲ = /a/
The Ashchenazi pronunciation is more close to /o/ but in the biblical Hebrew it was a kind of long /a/ maybe closer to the /o/ than the patach. These issue is in dispute, but no argue about the modern pronunciation....
Well, there are many opinions, but a symmetrical system of the Naqdanim with two es and two os (one open [ɛ]/[ɔ] and one closed [e]/[o]) seems rather stable and possible, like in Italian and French. The Tiberian קָמָץ is a merger of two Proto-Semitic vowels: short /a/ (דָּבָר < dabar) and short /u/ (כׇּל־ < kull). It is probable that these two had the same timbre, when the Tiberian Niqqud was added, the double pronunciation we use is old nontheless. Indication for the merger are word plays like יְחַלֵּץ עָנִי בְעָנְיוֹ Job 36.15, indication for the distinction is the Qumranic orthography, where ONLY originally short /u/ is sometimes written plene: בתומכי, i.e. בְּתׇמכִי. An argument for general open-mid back /ɔ/ (your second one) is the euphonic dagesh: לְכָה־נָּא analogous to forms like נַכֶּה־בּוֹ as a sharp unaccented syllable.
No, with long /a/ happened the Canaanite shift: שָׁלוֹם but original سلام (salām), or טוֹב for Aramaic טָב. When discussing קָמָץ, I thought the Tiberian stage of pronunciation was meant, where the vovel signs only indicate vowel quality, not length, as lenght has become purly allophonic: all short vowels in stressed syllables and open pretonic had lengthened. As for the example with the euphonic dagesh, next to the good argument that the vowel in כָּה should not change, when forming a closed, unstressed syllable כָּן with the dagesh of נָא to be still recognisable, the argument here for the exact quality of the phoneme, Chicenrunner has asked for, is maybe a weak one. If I understand it correctly (something is still strange with the opposition of a closed ạ and a close a̧), these cases with a dagesh euphonicus happen with the open one of vowel pair (cf. practically always הִנֵּה־נָא, but a close variety such as ạ would be rather unnatural just like the closed vowels ẹ and ọ.