All shipping methods that follow the Mageanto guidelines for building shipping methods will appear as an option when you try to set "Default shipping method". There are however a lot of Magento shipping methods that does not expose it's available options in such a way that it is picked up by this selector.

Often this is related to dynamic shipping methods that doesn't have a fixed set of options, but rather fetch them from an external source.

If you really need to set a default shipping method that isn't showing, contact us on support@onestepcheckout.com and we'll help you implement a fix for it.