*Coke* is pronounced /kəʊk/ in British English and /koʊk/ in American English. *Cock* is pronounced /kɒk/ in British English and /kɑk/ in American English.

As you can see, it is the vowel sounds that are different. The two sounds are distinguished in two ways: (1) by one being a diphthong and the other being a monophthong, (the vowel sound changes quality in a diphthong and remains stable in a monophthong), and (2) the position of the tongue is different. The “long O” sound of *Coke* is a diphthong, whereas the “short O” sound of *cock* is a monophthong, and it is pronounced with the tongue in a lower position. Here is a vowel diagram for British English:
![British English vowel diagram][1]

As you can see the vowel sound of *Coke* starts with the tongue in the position for /ə/, which is in a middle neutral position, and it moves up and slightly back to the position for /ʊ/. The vowel sound of *cock*, on the other hand, is pronounced with the tongue very low and very far back in the mouth, and it doesn’t move during the production of the sound. The differences in American English are similar.



  [1]: https://i.sstatic.net/MbPm8.gif