[qwerty-клавиатура]

s = '<a><b><ol>Хуй</ol></b></a>'; p = s.indexOf('<ol>');
p1 = s.indexOf('</ol>', p + 3);
console.log( s.slice(p, p1 + 5) );

document.body.innerHTML = document.getElementsByTagName('ol')[0].outerHTML;