I have talked to so many applicants who will absolutely never become a Marine Officer; and I can determine that in a matter of minutes. These are the trends I've seen that will absolutely tank your chances of becoming a Marine Officer.
Failure to take ownership of the application process
Your Officer Selection Officer (OSO) is very busy. While he/she will make every effort to contact you as often as possible, if you're not on top of your application there is a chance you will have missing documents when it comes down to the time in which you thought you were going to contract for a board. If you're not showing up to pool functions or weekly workouts and only calling once a month to follow up with the Human Resources Assistant (HRA), and Officer Selection Assistant (OSA), you are not making a memorable impression. Remember, your OSO has to write an evaluation about you. This can make or break you.
2. Making excuses for low PFT scores
COVID19 did not make you weak or ruin your run time. You did. Quit making excuses for a lack of self-discipline. Marine Officer candidates and 2ndLts are in fantastic cardiovascular shape. If you're not, you will have a tough time at OCS.
3. Failure to attempt to understand customs and courtesies
Officers who outrank you are referred to as "sir" or "ma'am." Enlisted are called by their rank. Learn the rank structure. While your OSO and OSA understand that you are not yet a Marine, you should act like you are. I still remember saying "yup" to my TBS (The Basic School) instructor. I very quickly understood the difference between "yup" and "Yes, Sir." Don't learn that hard lesson at TBS.
Customs and courtesies goes beyond just speaking appropriately to each rank. It is how you conduct yourself-- are you late to turn in documents or to show up to PT? Do you check your messages in the group chat? Are you responsive to the HRA? Remember, nobody cares more about your package than you do.
All in all, each of the pitfalls above boils down to a lack of self-discipline. To be an Officer in the Marine Corps you need to develop this trait NOW. Don't just scrape by with the bare minimum. Yes: you could slip through the cracks, but when you get in front of your first unit, you're gonna wish you hadn't.