Phonemically, there should only be a nasal vowel here, however nasal vowels are in the process of disappearing in Polish (compare the existence of nasal vowels with other Slavic languages). That's why you can often hear [ɔn] or [ɔm] (at the end) instead of [ɔ̃] (e.g. mądry, z mamą) and it is not considered a mistake. The same is the case with the letter ę ([ɛ̃]) - [ɛn] in the middle or reduced to [ɛ] at the end (e.g. między, robię).