Most schools will require a Bachelor's degree for admission, but that doesn't mean that it has to be completed before you apply. Many students apply for optometry school after their junior/3rd year and acceptance will be made under the condition that you complete your degree. Some schools will accept applicants without Bachelor's degrees, however it is rare, and you must have an extremely outstanding application. It is also common for people to wait until after completing their degree. In this case you could apply while taking some time off to gain more experience, strengthen your applications, work to save up money, etc.
It's not usually required that you complete all your pre-reqs before applying, but your acceptance would once again be conditional on your completion of those courses. However, it may be a good idea to get as many pre-reqs completed before applying/interviewing as possible, so the schools are better able to assess your application.
You should also consider the timing for when you want to take the OAT. That will also depend on when you want to apply (and I think most schools want it taken within 2 years of applying, but I'm not sure about that). If you plan on applying after your junior/3rd year, I would recommend taking it early in the summer before your senior/4th year. This would give you more time to focus on the OAT, provided you are not taking summer classes. And OptomCAS usually open up the application on July 1st, so if you get the OAT over with sooner, you will have more time to focus on your actual application. You want to get your application submitted as early as possible for whichever cycle you apply because optometry schools work off rolling admissions. Another thing that is important to know is that many schools have supplemental applications that must be completed in addition to the OptomCAS application!
You should also try to get some experience shadowing or volunteering with optometrists before you apply, as that is a requirement for many schools!
I hope that all makes sense! I tend to ramble sometimes, so let me know if I said something totally confusing!! You should also check out your school's Pre-Optometry Club if it has one!