oldstable is a codename for the previous debian stable repository, as long as security updates are provided. It is not just a name of the «Old Stable» version(s) of debian.
When the security update aren't provided anymore for 'oldstable', the repository is moved to http://archive.debian.org/.
# archive.debian.org
deb http://archive.debian.org/debian/ etch main contrib non-free
deb http://archive.debian.org/debian-security/ etch/updates main contrib non-free