Change the ol list code the Rich Text Editor outputs

<ol>
<li><p>item one</p></li>
</ol>

$e= &$modx->Event; 

switch ($e->name) {
           case "OnWebPagePrerender":
                $o = &$modx->documentOutput; // get a reference of the output
                $o = str_replace("<ul>","<ul><p>",$o);
                $o = str_replace("</ul>","</p></ul>",$o);
                 break;
           default :
                return; // stop here - this is very important.
                break;
}