It's not exactly my system; we work "on top" of it, developing new modules, maintaining installations and offering support and training, as partners of OpenERP S.A. We don't work on the core stuff.
That said, the addons I mentioned _are_ part of the core. OpenERP is an extremely modular system, where even some basic blocks are modules. But these modules come with the server, not with the "addons" set. See https://bazaar.launchpad.net/~openerp/openobject-server/trun...
Here's what I can find:
5000+ lines of ORM code: http://bazaar.launchpad.net/~openerp/openobject-server/trunk...
300 lines of tests: http://bazaar.launchpad.net/~openerp/openobject-server/trunk...
The ORM code is fairly shocking - really hard to follow, hundred line methods/functions and multiple one and two character variables.