Good Luck
If you are in shape, and feel your cardiovascular is in good shape, you are fine. OCS has the same physical demands as Marine Corps Boot Camp. In the 13 week cycle you will start with a 1/2 mile run in your 1st week, and increase a 1/2 mile every run until you reach the 5 mile mark. These are all formation runs with cadence, so it will not seem so tough. Additionally, you will do different physical fitness regimines every other day (with the running) to allow your body one day of rest, and to concentrate on academics. You will also have 2 physical fitness tests there as well, which, you will probably do your best on for your entire career, unless you are already a runner. If you are a runner, you will have no problem with the physical aspect of OCS. You will also have to attend The Basic School after OCS which is an extended training program to further your education as a Marine officer before you attend your occupational specialty training.
The basic difference between OCS and Ranger school or BUD's is that OCS is basic training for officers, and is meant as an indoctrination into the Marine Corps, and is a process to make new Marine Officers, not to break people. The other training, Ranger, BUD's, SF Q course, are all meant to weed out the many people who have exceptional abilities, but not the extremely exceptional abilities that those units need to accomplish their mission. Each one is a badass in his own way, who makes it through those courses, and never needs to toot his horn. Their accomplishment of the courses say enough.
Good luck to ya, and remember, the mind games are just there to make ya harder
Semper Fi