It doesn't sound like a diphthong to me. Maybe you need to practice the sound with something a little longer that ends in a nasal, like 'phone', 'loan', 'home', which all have the same 'o' sound. With the 's' of 'most', you tend to get to it too quickly.
Also, the 'm' of 'most' doesn't sound right. What's the first word you're saying? Is it the same?