Well, you could probably start on last years topic just to get your thoughts/accomplishments down on paper. If it's not the optomcas prompt, you can bet that one of the schools you apply to will ask a similar question. So it wouldn't be a total waste if you organized your thoughts in the form of an essay in preparation for other essays. If you hate writing, then you may as well start getting used to it now, because you're going to have a lot of essays on your plate once applications are released.
There's not much to compile in terms of the application aside from your essays. Just ask your college to send you a copy of your transcripts so that you have it ready when it's time to fill out the courses. You'll want an actual copy too (not the kind of thing online where you check your grades) because they want you to label each class exactly as it appears on your transcript and the titles are often abbreviated on the actual transcript. Might as well request your highschool transcripts too.
I guess you can also write down all your accomplishments, internships, extra curricular activities, work experience, volunteer work, etc. Then when the application comes out, you can just fill it in from your list.
I commend you for wanting to start early, but don't be frustrated! There's lots that you can do without entering it into optomcas. Ultimately, it takes like...3 hours max to fill out the entire optomcas with all your info/work info/experiences/class, so dont' worry, you're not losing all that much time. Even if the schools didn't use optomcas, I doubt their applications would be ready at this time anyways. 😛