I've known of people who applied later than December and have still gotten accepted, though keep in mind that their application must have been impressionable for schools to give them a spot. If anything, submit your application before and enter in your re-take date so that schools are aware you will be retaking. Taking the gap year is totally normal too if you don't get in the first time.
Not to be critical or harsh, but med school is no joke. The cost of applying is significantly much higher than optometry. Not to mention the process is longer so if you were to take a gap year, you'd be waiting for another year in between since you have to apply about two years in advance to enter immediately. (If you were to apply to med school now in your fourth year of uni, you'd be entering for Fall 2021.) So keep in mind that if you take a gap to figure out where you wanna go, for med school, it'd be 2 years before you start. Also, a AA 300 on the OAT is good, but it's not considered as competitive. You're at about the 50th percentile, so right smack in the average range. The MCAT is longer and much more vigorous than the OAT. I don't know how long you studied and what you did to prepare for the OAT, but just keep that in mind. Also if your TS was low, then the MCAT's science would be much more difficult.
Medical school isn't impossible to get in with low grades but it is significantly harder compared to the rising pool of applicants every year. If you truly want to do med school, prepare to re-take courses to raise your GPA. Even if you get in, if you found undergraduate courses hard, medical school may end up being very difficult for you.
If you're still having the what-if, try shadowing both ODs and MDs. You can still add the MD shadowing as experience and patient exposure if you end up applying to OD school. Try shadowing in hospitals as well to see which you enjoy. Ask questions to both ODs and MDs about life style, work balance, etc to see what seems like a fit for you.
Good luck on your journey though!
