Getting the parent organization unit (if any) of an organizational unit in Active Directory
public class OrganizationalUnit
{
    public string Name { get; set; }
    public string ParentUO { get; set; }
    public string Path { get; set; }
}

/// <summary>
    /// Provides an object that allows you to get organizational units within an
    /// active directory domain.
    /// </summary>
    /// <param name="connectionString">The LDAP connection string to a domain.
    /// For example LDAP://DC=YourCompany,DC=com</param>
    public ActiveDirectoryOrganizationalUnitRepository(string connectionString, string username, string password)
    {
        organizationalUnits = new List<OrganizationalUnit>();

        if (DomainExists(connectionString))
        {
            var baseDirectory = new DirectoryEntry(connectionString);
            baseDirectory.Username = username;
            baseDirectory.Password = password;

            DirectorySearcher searcher = new DirectorySearcher();
            searcher.SearchRoot = baseDirectory;
            searcher.Filter = "(objectCategory=organizationalUnit)";
            searcher.SearchScope = SearchScope.Subtree;

            var ouResults = searcher.FindAll();

            foreach (SearchResult ou in ouResults)
            {
                organizationalUnits.Add(new OrganizationalUnit() {
                                        Path = ou.Path,
                                        Name = ou.Properties["name"][0].ToString(),
                                        ParentUO = ou.Properties["parent"][0].ToString()}
                );
            }
        }
    }

ParentUO = ou.Properties["parent"][0].ToString();

OU=currentOU,OU=parentOU,...,DC=domain,DC=..

String myADSPath = "LDAP://onecity/CN=user,CN=Users,DC=onecity,DC=corp,DC=fabrikam,DC=com";
DirectoryEntry myDirectoryEntry=new DirectoryEntry(myADSPath, UserName, SecurelyStoredPassword);

Console.WriteLine("Parent is :"+myDirectoryEntry.Parent.Path);

DirectorySearcher searcher = new DirectorySearcher();

searcher.SearchRoot = baseDirectory;
searcher.Filter = "(objectCategory=organizationalUnit)";
searcher.SearchScope = SearchScope.Subtree;

var ouResults = searcher.FindAll();

foreach (SearchResult ou in ouResults)
{

    ResultPropertyCollection myResultPropColl;
    myResultPropColl = ou.Properties;
    Console.WriteLine("The properties of the " +
            "'mySearchResult' are :");

    foreach (string myKey in myResultPropColl.PropertyNames)
    {
        string tab = "    ";
        Console.WriteLine(myKey + " = ");
        foreach (Object myCollection in myResultPropColl[myKey])
        {
            Console.WriteLine(tab + myCollection);
        }
    }
}

organizationalUnits.Add(new OrganizationalUnit() {
                                    Path = ou.Path,
                                    Name = ou.Properties["name"][0].ToString(),
                                    ParentUO = ou.GetDirectoryEntry().Parent.Path}