Hii

Yes you can apply as OBC in JEE Mains application for april session as you are supposed o apply afresh application for April session even if you have appeared in January session.
But before selecting your category as OBC, make sure you have proper certificates as per the prescribd format of JEE athorities, so that you do not face any problem later. If you actually belong to Obc, then only go for that category and provide all the propoer documents for the same.

Hope this helps
Thanks!