Here's a quick tutorial on setting up Osclass location/category/user subdomains.
I've followed these steps on my sites and everything worked perfectly.

1. Log in to your hosting control panel (cPanel, hPanel, VestaCP, etc.)
2. Go to the subdomains section and create a new one.
3. Input "*" as subdomain name (without quotes!).
4. Login to your FTP and open config.php.
5. Change define('WEB_PATH', ' http://yourdomain.com/' ); to define('WEB_PATH', ' http://'.$_SERVER ['HTTP_HOST'].'/') ;
6. If required change " http://" to " https://".
7. Save the config.php file.
8. Log in to oc-admin.
9. Go to Settings -> Advanced.
10. Pick your subdomain type in the select box.
11. Save. That's it.

Additional notes:

If you have, for example, country subdomains (this will work for regions and cities as well), and you have a country named Croatia it's slug will be "croatia".
So the subdomain for Croatia will be "croatia.yourdomain.com".
But you want to only put the country code in the subdomain. Then go to Settings -> Locations, click on your country/region/city and replace the slug, for example, from "croatia" to "hr".