For restricting country to for example Australia, please
open up file
skin/frontend/base/default/onestepcheckout/js/oscgp.js

and add
componentRestrictions: {country: 'au'}

to all calls to
new google.maps.places.Autocomplete

so that it would look similar to this:
new google.maps.places.Autocomplete((funcField), {
types: ['geocode'],
componentRestrictions: {country: 'au'}
});