On the job training  is that ,your are been trained while doing your daily work at office, you learn at very step 
under the supervision of your senior. In  On the Job Training you are actually performing the task and while
doing it if you get stuck your doubt would get cleared right and then.

Advantages:
Practically performing
Proper Guidence by your supervisor while doing the task
Doudbt could be cleared

Disadvantage:
Theorytical knowlegde will be less
while propering the task you will come to know the process
Long Process and time consuming