Edit in JSFiddle

           $(document).ready(function() {
                if ($('ol:first').css('list-style-type') != 'none') { /* For IE6/7 only. */
                    $('ol ol').each(function(i, ol) {
                        ol = $(ol);
                        var level1 = ol.closest('li').index() + 1;
                        ol.children('li').each(function(i, li) {
                            li = $(li);
                            var level2 = level1 + '.' + (li.index() + 1);
                            li.prepend('<span>' + level2 + '</span>');
                        });
                    });
                }
            });

ol{margin:15px;}
html>/**/body ol { /* Won't be interpreted by IE6/7. */
                list-style-type: none;
                counter-reset: level1;
            }
            ol li:before {
                content: counter(level1) ". ";
                counter-increment: level1;
            }
            ol li ol {
                list-style-type: none;
                counter-reset: level2;
            }
            ol li ol li:before {
                content: counter(level1) "." counter(level2) " ";
                counter-increment: level2;
            }
            ol li span { /* For IE6/7. */
                margin: 0 5px 0 -25px;
            }

<ol>
            <li>List</li>
            <li>List
                <ol>
                    <li>List</li>
                    <li>List</li>
                    <li>List</li>
                </ol>
            </li>
            <li>List</li>
            <li>List</li>
        </ol>