1901 - Is it correct to read as Nineteen "o" 1 or Nineteen "zero" 1?

when you read 0230 - which is more appropriate - 'zero' two three 'zero', or "O" two three "O" (O pronounced 'Oh')

Depending on the answer above, what is the differece between the case where 0 is the number and O is the letter?? You decide..

Happy new year ...