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

On the job training imparts the training provide at the 
work place. Trainee will be learning the job and 
practically doing it at the same time. It implies learning 
and doing it at the same time..

Advantages:
Practical training under supervision
Clear doubts thn and there
Time gap of learning and doing can be minimised
Diadvantages:
Doing for the first time , error and risk is more.