This is a popular training program for the crunches part of the PFT.
While it is a good program to help you get to and past 100 crunches, remember that it crunches are a one-dimensional component of fitness and are not a substitute for a full core workout. OCS is much more than just the PFT.
Day 1
Do three maximum effort sets. Rest 60-90 seconds between each set. As soon as you stop (even for one second), that set is complete.
Day 2
Pyramid day. Begin with 20 repetitions. Each consecutive set should increase by ten until you can not complete the required amount without pausing. Then do a maximum effort set. Rest 1 second per crunch of the previous set.