1 There is different emphasis in these words: онА и Анна 2. "нн" sounds longer than "н" Тhat's all.

она - with short "o", almost "ана" Анна - double "н", "Ан-на"

Neither of these are actual words, but if you just want the pronunciation, unfortunately there isn't really a way to describe the difference. You would have to hear the difference. The best I can do is say that: OHA is oh ha and AHHA is ah ha the sounds are self explanatory, so I can't really describe it.

"анА" vs "Ан-на". "н" longer and stress is different.

I'm sorry, I thought this was for English