I wouldn't equate it to OCS. OCS is boot camp essentially with lots of leadership evaluation. Picture the full yelling thing with Dril Insructors. Leatherneck is more like a mini The Basic Scool (TBS). You will live, work and train in the TBS school, barracks and training area (it's about 15 minutes from the main side of Quantico). It's a lot of PT, hikes, land navigation (day and night so lots of map work, proper compass reading and learning terrain), squad and platoon tactics for offensive and defense operations in urban and non-Urban environments, weapons handling and proper deployment. There is also lots of intro to how the USMC works, traditions, culture, etc. along with fun stuff like shooting and blowing stuff up. Every year it can be slightly different but be prepared for something physical. The USMC PFT is 3 miles, pull ups and sit ups. The O course will be given. The e course will be run (6 miles in gear with obstacles). Not sure on hike distance but be ready for 6-10 miles ish with possibly 60 lbs (that isn't that bad). TBS is 6 months long and moved up to company and battalion tactics. Leatherneck won't go that far but is a good intro to the building blocks to that. It really prepared me well for TBS.
During all this Mids will be given leadership positions in and out of the field to evaluate leadership ability. So a common one would be a squad attack. They look at how they prepared, deliver the op order, how it was executed, how they handled adversity during it, etc. The staff is usually a Capt or Major as a Platoon leader from USNA. Usually 1-2 2ndLts are also assigned ago help teach, train and evaluate.