Which charset does your Oracle database use? Pls open the file koolreport/core/src/datasources/OracleDataSource.php and replace the following line:

    $conn = oci_connect($username, $password, $connString, "", $session_mode);

with this one:

    $conn = oci_connect($username, $password, $connString, "UTF-8", $session_mode); // if your database uses a charset other than UTF-8 replace "UTF-8" with it.

Ok, then pls try "AL32UTF8" and see how it goes:

    $conn = oci_connect($username, $password, $connString, "AL32UTF8", $session_mode);