'''OneStepCheckout works fully out of the box with 99% of the payment methods that exists for Magento!'''
With over 19,000 OneStepCheckout installations, we have seen all kinds of payment methods out there. Luckily, almost all of them works perfectly right out of the box with OneStepCheckout, without requiring any further customizations. All modules that are following Magento guidelines for developing custom paymnet methods will just work.
There are however always a few modules that has done things differently and therefore isn't supported by OneStepCheckout. In most cases, we can have a look for you and try to resolve the issue, but if it requires too much time to get it sorted we have to charge consultancy hours for making them integrate with each other.