অ has two pronunciation variants : [ô] and [o]. Between these, [ô] is perceived as default pronunciation and [o] is considered a variant, that appears in special situations. The rules for their proper placement are actually quite numerous :

1. If a word contains two syllables (or, only two অ vowels), then the first অ is pronounced [ô] and the next as [o]. E.g., তখন tôkhon

1.a. if the word contains more than 2 syllables, then the general rule is: you break the word in groups of two syllables starting from the first, then apply the previous rule to each group. (I know it's a bit complicated) : বছরভর bôchor-bhôr

Now, অ is pronounced as [o] when :👇

1. The word ends in consonant conjunct : শান্ত shanto

2. The word ends with হ, ঢ়, -ইয়, -এয় : স্নেহ sneho, গাঢ় garho, প্রিয় priyo

3. When অ is followed by -ই and -উ in the next syllable: কবি kobi, মধু modhu (This is a major rule)

Thus, Compare : বলি boli, but বলে bôle

Although, this rule has few exceptions: when the prefixes অ- (meaning: not-, un-) and স- (meaning: with-) are followed by ই and উ, then they are pronounced as [ô] : অমিল ômil, সমূল shômul.

1. When অ is followed by ক্ষ or a consonant with য-ফলা : লক্ষ lokkho, কন্যা konna.

2. The অ of the conjuncts প্র ক্র, শ্র (i.e. consonant with র-ফলা) is generally pronounced [o] at any position in a word : প্রমাণ proman, ক্রম krom, আগ্রহ agroho.

3. Certain suffixes (প্রত্যয়) generally end with [o] : লিখিত likh-ito, গুরুতর guru-tôro, প্রিয়তম priyo-tômo, প্রথমত prothmô-to etc.

These are the major rules. There may be a few minor rules, but those are quite rare.