With apache2, it is called a redirection (see http://httpd.apache.org/docs/current/rewrite/remapping.html or http://httpd.apache.org/docs/current/mod/mod_alias.html and http://en.wikipedia.org/wiki/URL_redirection )
You should also read http://httpd.apache.org/docs/current/mod/mod_proxy.html which is nicer (the reverse proxy).
A reverse proxy (or gateway), by contrast, appears to the client just like an ordinary web server. No special configuration on the client is necessary. The client makes ordinary requests for content in the name-space of the reverse proxy. The reverse proxy then decides where to send those requests, and returns the content as if it was itself the origin.
A typical usage of a reverse proxy is to provide Internet users access to a server that is behind a firewall. Reverse proxies can also be used to balance load among several back-end servers, or to provide caching for a slower back-end server. In addition, reverse proxies can be used simply to bring several servers into the same URL space.
A reverse proxy is activated using the ProxyPass directive or the [P] flag to the RewriteRule directive. It is not necessary to turn ProxyRequests on in order to configure a reverse proxy.
Just an example, assuming you need to fake www.my-domain.tld/opendoo to https://my-company.odoo.com :
# Reverse Proxy for Odoo
ProxyRequests Off
SSLProxyEngine on
<Proxy *>
Order deny,allow
Allow from all
</Proxy>
ProxyPass /opendoo https://my-company.odoo.com
ProxyPassReverse /opendoo https://my-company.odoo.com
Not tested but should work...
CNAME is not an option : it'll give the good IP address but the Odoo's server is probably full of vservers, so without the correct address (my-company.odoo.com) it will fail.