on time x in time

On time" significa "na hora certa, ou seja, não estar atrasado".

Exemplos:

I hate when people aren't on time.

Our flight is on time - great!


"In time" significa "em tempo para fazer alguma coisa; antes do previsto".
Exemplos:

We had to hurry, but fortunately we got there in time to get good seats.
Our flight is on time - great! We'll get there in time to do some shopping.