When does the O sound like an A

[terhje]

Hey all , I've noticed that the O can both sound like and O and an A.
For example: In "его" it sounds like a normal O, but in" преподавтел" it sounds like A.
can someone please explain?

[кальмар]

It all depends on if it's in the stressed syllable or not. If the O is stressed it sounds like O. if it's not stressed it sounds more like A.

For example, она (she) sounds like [ana] because O is unstressed.

облако is pronounced as [ˈobləkə] because stress falls on the first O.