Some users of Osclass, sooner or later they will move away from the discontinued Osclass classifieds script, unless some miracle happens.
The alternatives are either a standalone php classifieds script or a Wordpress plugin/theme.
It is better to know what your options are, so here is a list and i will update it when something new is added from @tycho-brahe :
FREE Standalone php Scripts:
https://www.valueweb.gr/forums/osclass/yclas-free-alternative-php-classifieds-script/
FREE Wordpress plugins and themes:
https://www.valueweb.gr/forums/osclass/wordpress-based-classifieds-plugins-and-themes/
________________________
* You can create a SUB-domain in your hosting plan and install in there evetything you want to test. It is completely separated from your main site and you can do whatever you want, without disturbing your Osclass production installation.