No. OBC and EWS are 2 different Categories and so you cannot apply for EWS Category Certificate if you have OBC Certificate. EWS Reservation is for those Candidates who belong to General Category and whose family annual income is less than 8 lakhs per annum from all sources. And these are the Candidates who do not claim any OBC,SC,ST benefits. But because you belong to OBC and are availing its benefits, you cannot qualify under EWS Category.