I've been searching all day how to make offer visible for all statuses no matter if an advertiser chooses free - price field or check with seller.
If the checkbox is checked then the make offer link is visible on all statuses and if the checkbox is of it shows no make offer link.
It seems like it's logic itself, but how to put it into practice? Is this easy to change in plugin files?

Solved by editing code in plugin make offer.
In index.php:
Find: if(osc_item_price() !=0) {Change line to: if(osc_item_price() !=1 || (osc_item_price() !=0))  {

@mwindey
For me make offer on listing that is "Free" does not make sense at all, as best option is already available - free.