An OSO, Officer Selection Officer, is a recruiter for officers. They work at OSS, Officer Recruiting Stations. An OSO is almost always a captain. Enlisted recruiting stations can be found in almost every town or city. This is not the case with OSS. If you live on the East Coast, you may have one ten minutes from your house. If you live in South Dakota, the closest one is in Wyoming. This one services many states.
Getting selected to be an officer is a long process. Your OSO doesn't determine if you go to OCS. They determine if you meet the qualifications, then submit your "package" for a review board. It is in an OSO's best interest to invest their times in candidates that have a better shot. However, the initial meeting is just that. An initial meeting. This meeting is primarily a way for your OSO to guage if you will one day be competitive, and how long until your package is ready to submit.
Your OSO may make you take an informal PFT, or maybe not. This is to guage where you are in preparation. If you fail, it's not end game. I failed mine miserably, but it let my OSO know that pull-ups were my problem, not running. A PFT is made up of:
Pull-ups (not timed)
Plank
3 mile run
Here is the Marine Corps PFT page. Crunches are being phased out.
I will find the plank score chart. 3:45 is perfect score.
As much as you want to be a Marine officer, your OSO also needs to recruit people. So don't be intimidated. Dress business casual, bring a notepad and pen, and some questions. Know your timeline. When do you graduate? What contracts interest you? Active or reserves? Things like that. This is just an initial meeting, so don't sweat it.