Posted 
We are happy to announce that OneStepCheckout from version 4.1.0 released two weeks ago is fully responsive, and works great in all devices (smartphones, tablets, desktops).
Existing customers can simply download and upgrade to see the new benefits. Please see more details about how to activate the responsive features in our wiki.
Also please keep in mind that you should always test new versions of OneStepCheckout in a non-production environment before trying to upgrade your actual store with actual customers.