2ndLts and OSOs alike offer loads of advice prior to attending OCS. I heeded no advice or warnings and suffered deeply as a candidate. Be humble and accept the advice of others. It's healthy to be a skeptic but when your skepticism becomes arrogance, you suffer.
Study student outlines prior to showing up
All of the information you will be tested on is at your finger tips. I guarantee that your OSO has all of the student outlines. Ask for them and study them religiously. This will give you the ability to sleep longer, keep your mind sharp and give your body time to recover. Unless you want to stay up until 0100 every night studying your student outlines with a red light headlamp, heed this warning.
2. Learn how to climb a rope properly
Use the J-Hook method. It's the quickest and easiest method to climb a rope. The S-Method may feel safer, but it's slower. Find a rope to practice on. Don't let something as simple as climbing a rope ruin your chances of passing the obstacle course.
3. Hike
Hike on your own when you're home. You don't need an Individual Issue Facility (IIF) USMC Pack, but if you can afford to buy one to hike with, go for it. At the end of the day, the goal is to show up to OCS with broken-in boots and hardened feet. You should have already worked out what you have to do to prevent blisters as best as possible BEFORE showing up to OCS.
Look: at the end of the day nothing at OCS is that ground-breaking. It's just all new and ominous to you because... well.. you're new to this. Colin Powel said "there are no secrets to success. It is the result of preparation, hard work, and learning from failure." If you come into this gun-club being a person who prepares, you'll be successful not just at OCS but as a Marine Officer.
Semper Fidelis