Hi arzidan,
This is possible with Osclass. I have tried it quite a while ago but I just tried it again.
What you have to do is install Osclass in a sub folder then copy the index.php file back to the root folder then edit the index.php on line 22
This is the default code
define('ABS_PATH', dirname($_SERVER['SCRIPT_FILENAME']) . '/');
Change it like so
define('ABS_PATH', dirname($_SERVER['SCRIPT_FILENAME']) . '/sub-folder/');
change the word sub-folder to what ever you named your sub folder. Then when someone comes to your site using your root url they will see your site.
Jay
Jay