The pronunciation of Russian vowels varies according to the stress. Stressed vowels have their full value. Unstressed vowels are usually reduced in value. This is particularly noticeable with the letter "o".
Check with the recording:
вино́, вино́, вино́ (wine)
хорошо́, хорошо́, хорошо́ (good)
пробле́ма, пробле́ма, пробле́ма (a problem)
How does the letter "o" sound:
When stressed?
When one syllable away from the stress?
When two syllables away from the stress?