This should work:
Dim sCn As String = result.GetDirectoryEntry().Properties("distinguishedname").Value.ToUpper
Dim iLength As Integer = InStr(InStrRev(sCn, "OU="), sCn, ",") - (InStrRev(sCn, "OU=") + 3)
lblOU.Text = Mid(sCn, InStrRev(sCn, "OU=") + 3, iLength)