Google started complaining about the missing „priceCurrency” field in „offers” . It looks like that the warning appears when the ad is free -- it costs 0. Looking through the code I found the following in oc-includes/osclass/structured-data.php :
<?php if(osc_item_price() > 0) { ?>"priceCurrency": "<?php echo osc_esc_html(osc_item_currency()); ?>",<?php echo PHP_EOL; } ?>
So it looks like that the IF should be removed to satisfy google and the currency should always be printed even if the price is 0. Like with the price field -- it's always printed.
I'm guessing the line should look like this:
"priceCurrency": "<?php echo osc_esc_html(osc_item_currency()); ?>",