Sounds and letters - two different things in English

The letter of the alphabet 'O' is pronounced /əʊ/ in many words.  

 e.g.  global → / əʊ /          

Most of the time it is pronounced /ɒ/.    

 e.g.  hot , module→ / ɒ /   

But it can also be pronounced /ʌ/    

 e.g. brother → / ʌ /
        
When you have a combination o  +  consonant +  vowel, you pronounce it / əʊ /.    

Notice the difference between the words below.
e.g.   not→ / ɒ /                note→ / əʊ /   
         cod → / ɒ /               code → / əʊ /
         hop → / ɒ /               hope→ / əʊ /

That is why you have to double the final letter in comparative forms not to change the pronunciation: e.g. hot → hotter

▪ If you add 'r' after the 'o', the pronunciation is different again, as you pronounce it /ɔː/   

 e.g.    storm, torn → / ɔː /              

But it can also be pronounced / ɜː /   

  e.g.     world, worse, work → / ɜː / 

▪ If you have a combination of two letters o, it is pronounced either / ʊ /   

  e.g.   good → / ʊ  /                      

or / uː /   

    e.g.     food → / uː /

A third pronunciation is possible though it is much less common: /ʌ /   

  e.g.   blood → /ʌ /