Since you have a 3.9 you must have good grades in your other science classes. It is a summer class so it goes quickly and organic chem is really difficult to learn even over a semester. There is so much to learn. The C is understandable. But, since you probably have good grades in your other sciences so far, this does not ruin your chances...
Just study like hell for organic II and get an A when you take that. If you do, the C will not look so bad. Also, take more upper level science classes and make sure you do well in them and the C will just be a small blemish. Also, when you take the DATS, put extra effort into organic chem and it will show the D-school that the C could've been just because of the short semester or because it is Harvard and the prof is difficult.
When you take Organic 2, I suggest studying for exams a week in advance. I found that organic chem takes time to click. If you start very early, you will be able to start learning all the material and have enough time for it all to make sense because I found that sometimes even after 20 hours of studying, I knew nothing. Yet, the next day I would wake up after a good amount of sleep and everything would click. Practice tons of reactions. That's all it takes, starting early and practice practice practice. You definitely did not ruin your chances, do not worry about that, just try harder 🙂.