Try to focus on the task at hand. Take online practice tests to familiarize yourself with the test! Kaplan and Princeton Review OAT prep books come with practice tests for you to take (and if you're doing poorly on them and that's worrying you, remember that those practice tests are usually MUCH more difficult than the real thing). Pushing back your application two weeks will not make too much of a difference. You need to go into your OAT feeling ready and confident in your abilities, otherwise you will set yourself up for failure (mind over matter is real!).
Others have mentioned that you are still able to submit your application without having taken your OAT yet. You are also given the option to say in your application "when" you will take the test, so the school will be able to see that you are at least due to take it soon. One thing that concerns me is that you haven't been focusing too much on your application, in order to focus on studying for the OAT. Don't forego putting the appropriate amount of time into your application just for studying. The OAT is important, but the quality of your answers to essay questions and your personal statement are just as important, if not more important! The OAT is a standardized test, which compares you to other test-takers. But your essays will allow the committees to see who you are as an individual in your own words! If there are schools you are applying to which don't have a supplemental application, and your personal statement is completed, submit now!