The characters OD in contact prescription has the same meaning with eye prescriptions. It stands for right eyes while the OS stands for left eyes. In fact, it is Ok even if you have no idea what it stands for. You can just give your prescription to optican who can provide you proper contact lenses.
It is the right eyes. And the OS is your left eyes. And it is normal for some people have higher power in one than the other. It is not a big problems, you don't worry about it too much as long as the prescription for your both eyes are not different too much. Anyway, you shall keep a healthy life style which may help you reduce the different in prescription for both eyes. Hope this can help you.