The following code:

HtmlDocument doc = new HtmlDocument();
doc.Load("test.htm");

Console.WriteLine(doc.DocumentNode.SelectSingleNode("//th[starts-with(normalize-space(text()), 'Oms')]").InnerHtml.Trim());

will dump this:

Oms (tkr)

But you'll have to parse the end manually. The Html Agility Pack only knows about elements and attributes. The XPATH expression means: select any TH element that has a text content that starts with 'Oms', once trimmed (normalize-space).