It is never an [o] sound. Even when an [ʌ] sound is spelt with a letter 'o' , as in 'sometimes' or 'money', we never pronounce it as an [o]. There are regional variations, of course. Many people in the north of England pronounce the [ʌ] sound as [ʊ] - the sound in 'put' and 'book'. So 'luck' would be pronounced the same as 'look', for example. I wonder if you were hearing [ʊ], and interpreting it as [o] ?