To get started with OSE-Omni, you'll need to get familiar with Git and Repo.
To initialize your local repository using the OMNIROM trees, use a command like this:
repo init -u git://github.com/OSE-Omni/android.git -b ose
Then to sync up:
repo sync
Then to build:
cd <source-dir>; . build/envsetup.sh; brunch <device_name>