Drush makes it very simple to create a subtheme. For this you need to download, install, and enable the Omega Tools module first (this should be done anyway as Omega Tools has features outside Drush such as theme exporting). Once you have Omega Tools and Drush installed and configured, perform the following steps.
Create a subtheme with Drush
From a shell console in your site’s root directory (not being in your site root can cause problems in the Drupal installation) type
drush omega-subtheme mysubthemename where mysubthemename is the name you want to give your custom subtheme. Don’t use any dashes or spaces in the subtheme name, or make sure to enclose the subtheme name in quotes. If you are running a multi-site setup and you want to create a new subtheme in a particular site's themes directory, navigate to sites/mysite and execute the command
drush omega-subtheme "My Theme" --destination=mysite.
Your subtheme will by default be based on the Omega HTML5 starterkit.
To base your subtheme on the Omega XHTML starterkit use the option "starterkit":
drush omega-subtheme mysubthemename --starterkit=starterkit_omega_xhtml
For all other options enter:
drush help omega-subtheme