When to use ‘on time’ and when to use ‘in time’?

‘On time’ means ‘at the planned time’; that is, neither too early nor too late. The opposite is ‘early’ or ‘late.’ It is often used to refer to timetabled events.

Example:

I just checked with the airport; the flight is on time.
I want the meeting to start exactly on time.

‘In time’ means ‘with enough time to spare’ or before the last moment. Its opposite would be ‘too late.’ 

Example:

He would’ve been dead if you hadn’t got him to the hospital in time.