EWS and OBC are alltogether 2 different Categories and so you cannot apply for both at the same time. EWS reservation for those Candidates belonging to General Category and whose annual income is less than 8 lakhs per annum and also those  who are not availing reservations of any other Categories like OBC,SC,ST. So if you already belong to OBC Category, you cannot claim EWS reservation. This is also because you do not belong to General Category.