If i understand right, you want to change the free text or make some changes on that level free?
You don't have to change the core files, you have some filters in formatted price that you can use.
For example for changing the free tag with the price and symbol:
this in your theme functions.php file
function new_filter_for_free_price(){
return osc_item_price().' '. osc_item_currency_symbol();
}
osc_add_filter ('item_price_zero', 'new_filter_for_free_price');
Of course that you can play more with this
function new_filter_for_free_price(){
return __('your text');
}
In this way you don't have to make this changes every time you update osclass.
You have more filters there that you can use in the same way.
osc_add_filter ('item_price_null', 'your function' ); - for check with seller
osc_add_filter('item_price', 'your function' ); - for final price