Sorry if I'm misinforming here, but I think you cannot go directly for the 1z0-816 exam. You have to take the first exam (1z0-815), then take the second exam (1z0-816) to receive the OCP: Java SE 11 certification (actually, you could take the 1z0-816 exam first, but I'm almost sure you would be forced to take the 1z0-815 in order to receive the certification).
Look at section 3 in the following document, provided by Oracle.
Java SE 11 Certification: Questions Answered
So, given that you would have to take 2 exams anyways, I would suggest taking the 1z0-809 exam first, carrying the momentum you have now. When you pass this exam you get the certification for OCP: Java SE 8, which it still quite an achievement.
After that, you can go for the upgrade exam.
Best of luck!