Get all ous in active directory in hierarchical order

Domain domain = Domain.GetDomain(new DirectoryContext(DirectoryContextType.Domain, "domainname", "username", "password"));
Forest forest = domain.Forest;
DomainCollection domains = forest.Domains;

List<string> lstName = new List<string>();
            DirectoryEntry directoryEntry = new DirectoryEntry("LDAP://DC=domainname,DC=com", "username", "password");
            DirectorySearcher oDirectorySearcher = new DirectorySearcher(directoryEntry,"(objectCategory=organizationalUnit)", null);

            SearchResultCollection oSearchResultCollection = oDirectorySearcher.FindAll();
            foreach (SearchResult item in oSearchResultCollection)
            {
                string name = item.Properties["name"][0].ToString();
                lstName.Add(name);
            }