Hey Jignesh,
The selection procedure for OFFICERS TRAINNING ACADEMY  through combined defence services examination  is as follows
1.applying for the written exam after fulfilling the eligibility criteria
2. aspirants don't have to appear for maths paper ,they just have to appear for english and  GK ,after clearing the written , comes the dates for service selection board
3. On clearing the test in service selection board , now the candidate has to go through a series of Medical checkup , if the candidate is declared fit by the medical board , results in his final selection
4.now all the candidate has to do is to wait for the merit list , if he makes the cut , then he shall receive call letter for the academy
Hope this helps