If you are graduating or have already graduated, then you can't use CPT. CPT is for those students who are still in college and who want to work or do internships on areas *closely* related to their courses. But if you have already applied for OPT and have the receipt, then you can still work in the company but you will have show the proof that you have applied for OPT to the HR people at your company--by submitting your receipt. But again, it depends from company to company; some might agree to it, some might not.