Hi Samanpreet! If your query is about applying for CAT or other MBA entrance exams, then you must have a valid certificate at the time of application. You need to upload the certificate as well as the proforma (available in the official CAT website). 

Hi,
If you claim that you are an OBC candidate there is nothing wrong with this, but when concerned institute will need that certificate to check your candidature you should be able to produce certificate, only then that institute will admit you as an OBC candidate. I hope it helps!