Officer Candidates School is the interview process. It is designed to weed out those who do not posess the leadership abilities required to lead Marines into a combat zone. It is 10 weeks of no sleep, physically and mentally challenging problem solving scenarios, and extreme physical conditioning. They will yell at you and force you to make difficult decisions without sleep, or a full understanding of th environment around you. This is essential because in combat you will never have the whole picture, and your ability to make a sound decision without all the information is critical. No decision is the worst course of action you can choose. You will be put in command of your peers, and expected to lead them. Egos and personality types will clash, almost everyone around you is an alpha male. You will fail. They want to see how you handle failure, how you rebound. You have to rate your peers, and they have to rate you. You will see every evaluation your peers make on you. Pride cannot get in the way of poor leadership traits you posess that you must correct. Your Marines' lives depend on your ability to lead them. Integrity is the number one trait you must posess. Your word will be the word of God to your Marines. What you say goes, therefore you must speak honestly, and never look the other way. You must overcome the stigma of being a "rat" if you witness someone else lying, and report it. You will be held just as accountable as the liar or cheater if you do not.

Tests, medical, etc I don't remember the entire process so I don't want to do any guesswork. It's long and it takes a few months. You can get more information from a recruiter.