how to store tnsnames in LDAP or ActiveDirectory

A very good tutorial : http://oracle-cookies.blogspot.com/2007/01/get-tnsnamesora-from-openldap.html

Please be aware, that , if you have SUSE or OPENSUSE, it is better to user YAST2 in order to configure openldap , rather than editing directly /etc/openldap/slapd.conf

Also: you can check the following links :
 http://blog.ronnyegner-consulting.de/2009/09/30/storing-oracle-database-connection-strings-in-openldap-instead-of-oid/
http://laurentschneider.com/wordpress/2011/11/tnsnames-and-active-directory.html
http://eldapo.lembobrothers.com/2009/05/19/using-any-old-ldap-server-as-an-oracle-name-server/