Odoo can be found on Github, you can find Odoo 11 here.
You can find the official downloads (for Windows, Debian, RPM and the sources) at https://www.odoo.com/page/download 
To install it on Ubuntu you can follow the following steps:

1. Download my installation script from Github and copy it locally:

sudo wget https://raw.githubusercontent.com/Yenthe666/InstallScript/11.0/odoo_install.sh

2. Edit the file and modify the parameters that you’d like to change:

sudo nano odoo_install.sh

Tip: If you’d like the enterprise version you should set IS_ENTERPRISE to True instead of false.
3. Make the file executable:

sudo chmod +x odoo_install.sh

4. Execute the script and wait a few minutes for Odoo to install:

./odoo_install.sh