It's quite interesting issue that is on Osclass probably for very long time.
Problem is that we use urlencode to pass category slug to mysql, that is wrong (chars are escaped).
Go to oc-includes/osclass/model/Category.php
around line 524, find:
$slug = urlencode($slug);
Replace it with:
$slug = $this->dao->connId->real_escape_string($slug);
$slug = urlencode(urldecode($slug));