They often mean the same, but sometimes they're different.

'On time' means 'according to a schedule or a deadline.'

If I don't finish my work on time, my boss will be angry.

My train is never on time.

'In time' means 'not too late to be successful.'

I missed my deadline, but I still got the presentation finished in time for the meeting.

My train was late but it still arrived in time for me to get to work early.

Neither my presentation nor my train were on time, but they were in time for me to do what I needed to do.

It's good to be on time.

It's even more important to be in time!

The difference is down to how flexible your schedule is.

But be careful!

Sometimes if you're not on time, you won't be in time either.