Will I get rejected during a medical test of any multinational private companies in which I am going to do a job as a mechanical engineer if I have a deviated nasal septum?

No, unless your DNS is so severe (deviated nose bone) that you are hardly able to breathe.

If your DNS has not caused or leading to any other condition like sinusitis, asthma, rhintis, ear infection, tiredness, lung problems, you should be fine.

I am not trying to scare but if you dont breathe properly due to DNS it will affect overall health.

Thing is there are people who have severe DNS but dont have any problems. They are healthy overall and physically active.

Thing is DNS is a manageable condition but it requires continous effort in severe cases. Else with age it can cause health issues.