Each domain needs an individual OneStepCheckout serial key to have a valid license.
When you purchase a brand new OneStepCheckout License, go to the top right section of your order form, simply choose the number of production domains you need and type in the text field below their urls.
If you have already purchased a OneStepCheckout License, you can purchase Additional Domain License, as long as the new domains belong to the same legal entity as the domains listed in the original order. Here is where you can purchase those additional domains: https://www.onestepcheckout.com/additional-domain-license.html
If you purchased a OneStepCheckout License before 30 June 2017, email email@example.com for your serial key requests. Please make sue the new domains belong to the same legal entity as the domains in the original order.
More questions about Licensing and Serial Keys? Contact firstname.lastname@example.org