There's a way to enable generating micro data for google, facebook, twitter in the Oslcass settings. This adds also the rating to the posted listing. Unfortunately when no one rated the listing yet, it shows 0/5 (instead of some text like "not rated yet"). I think this should be fixed because if you use this option, everyone will see in the google search that your listing is very badly rated.

How can you tell google that product is not rated?

I see it in google search results. Also there are issues with this rating on google search console. And it threats this as really bad warning preventing the ad to be visible in google search if someone reports it. So I disabled this generated microdata for now.

I think I know where the problem is:

"ratingValue": "<?php echo round(osc_count_item_comments_rating(), 1); ?>",
This returns 0 when there's no comments. And that's why Google shows 0 in its rating.

When I posted a coment and rated 5, it showed 5. So something is clearly wrong here.

@morfik
Then google will start complain you are missing these micro data.
There is no ultimate solution, google is wrong and require rating even there is no rating.

Yes, I know, I see the warnings, but having 0/1 rating on every listing is really bad. I don't remember, but when I was using Wordpress long time ago, it didn't have this issue with 0/1 rating. It simply said no reviews/ratings yet.

So maybe it would be better to print 5 instead of 0 where there's no comments/ratings? What's the clever way of doing it?

@morfik
That's even worse.

Why? It would be showed in google as 5 instead of 0, why is this worse? We can assume that admin likes all the ads equally and gives them 5 after they were posted. 

@morfik
Because you provide false data to google just to make your site looks better.

But if it's 0, it also provides false data to google... If I had to choose, I would stick with 5. So how to change it?

Core or item.php

I'm not a programmer, so what should be put instead of:

"ratingValue": "<?php echo round(osc_count_item_comments_rating(), 1); ?>",

Basically how to compose a condition like:

if there's no comments, print 5,  else print what should be printed 

osc_count_item_comments_rating() > 0 ? osc_count_item_comments_rating() : 5

I have no idea how to rewrite the following line 

"ratingValue": "<?php echo round(osc_count_item_comments_rating(), 1); ?>",
in

  "aggregateRating": {
    "@type": "AggregateRating",
    "ratingValue": "<?php echo round(osc_count_item_comments_rating(), 1); ?>",
    "bestRating": "5",
    "worstRating": "0",
    "ratingCount": "<?php echo (osc_count_item_comments() > 0 ? osc_count_item_comments() : 1); ?>",
    "reviewCount": "<?php echo (osc_count_item_comments() > 0 ? osc_count_item_comments() : 1); ?>"
  }

osc_count_item_comments_rating() > 0 ? osc_count_item_comments_rating() : 5

Its there. Or its different question?