What Does Odoo stand for ...?
Short Answer: It does not mean anything.
Long Answer: When we had to choose a new name for TinyERP, we purchased two domains: OpenERP.Com and odoo.com! We purchased both, just in case and we chose the wrong one (OpenERP).
A few month later, we discussed about using odoo.com for the online offer of OpenERP, but we never did it in order to not split the brand. (Odoo could have been an acronym of On Demand Open Object). OpenObject was the name of the OpenERP framework at that time but we don't use this name anymore too.
The official, and more funny, answer is here: https://www.odoo.com/blog/5/post/156