These expressions are very similar, but there is a slight difference in meaning.
On time means that there is a specific time established when something is supposed/expected to happen, and it is happening at the planned time.
- I want to start the meeting on time so please don’t be late.
- The 11.45 train left on time.
If we say we arrived in time, we’re saying that we got there a little early – that there was time to spare.
We often use the structure in time to do something:
- We arrived in time to get some drinks before the show started.
- Will you be home in time for dinner?