OC and UR are one and the same. They are the General Category in which there is no reservation and Candidates belonging to to this Category get admissions based on merit. EWS is a Category born out of UR. EWS Category is a Category which is for those Candidates who belong to General Category but whose annual income is less than 8 lakhs from all sources. So if you belong to General Category and have annual income less than 8 lakhs per year, then you are eligible for EWS Category.