I’m going to guess that pronouncing 0 as “o” takes less effort to say than zero for most English speakers. You would hear it when time is read out (for the first nine minutes after the hour though). For example, 09:01 is pronounced “nine o one” and not “nine zero one”. You would also hear 0 pronounced as “o” in phone numbers. You would also hear it in any title ending in 101.

En general, se pueden decir los dos. Solo hay dos excepciones que me vienen a la mente en este momento: 1) cuando se cuenta (0,1,2,3...) y 2) cuando se precisa la cantidad. En estos dos casos, “zero” es la única posibilidad. Por ejemplo, no se puede decir “I have o siblings”, pero sí se puede decir (y se debe decir) “I have zero siblings.” Espero haberlo dejado claro. Avísame si tienes preguntas.

Why? I don't know. But some people speak that way. In my region, we say "highway four oh one" not "four hundred and one" or "four zero one". We have some telephone numbers that start with "nine oh five" not "nine zero five". But if it is something technical (temperature, speed, a long number), we say "zero".