Hello Bapina, to apply for UPSC exam in the OBC category, there is a form of certificate required by the Government of India. The pro-forma for the certificates required are given on the official site of Union Public Service Commission (UPSC). You need to check whether your SEBC certificate conform to the Proforma provided.
Also, I would like to tell you that, there are two Proforma, one for claiming that you are from OBC category, and the other is for declaration by the OBC candidates.
You can check it through this link,
https://upsc.gov.in/recruitment/recruitment-performas
Further, the examination notice issued by UPSC for civil services examination states very clearly that, "the candidates are not required to submit any certificates along with their applications in support of their claims regarding age, educational qualifications, Scheduled Castes/ Scheduled Tribes/ Other Backward Classes/ Economically weaker sections and Persons with Benchmark disabilities etc. which will be verified at the time of main examinations only". (On page 18, how to apply, note 6, final notice for Civil services examination). But these proofs will be required at the time of counselling.
So, the last date to submit the online applications for Civil services examination is 18th March, 2019 till 6 P.M. after that the link will be disabled.
Also, I would recommend that you take the following documents with you to the exam, as they are required:
Admit card
Adhar card
Driving licence
Passport
Voter ID
Two passport size photographs
The tentative date for receiving the Civil services examination preliminary exam - admit card is 15 May,2019. And the exam will be conducted on 2 June, 2019.
The documents that will be required at the time of counselling are:
1. Birth certificate or high school mark sheet.
2. Printout of e-summon letter.
3. 10 and 12 class marksheet
4. Graduate/post-graduate/other examinations passing certificate.
5. Passport size photograph with one attached to the interview call letter
6. Character certificate
7. Caste/community certificate issued by the concerned authorities to SC/ST/OBC candidate.
8. In case the candidate is physically challenged, he/she must bring their medical certificate.
9. A discharge certificate in applied for an ex-servicemen.
10. Adhar card/ passport/ Pan Card/ Voter ID or any other photo ID card issued by a statutory authority with a clear photograph on that card.
All the best!
I hope that helps.