How the RTL css system works in drupal specially in omega theme?
The css file with the rtl at the end will be loaded whenever you have a RTL language added to your site. in Omega as the CSS files are generated by Scss you need to create a scss file with the rtl at the end of the file so at the time of compiling the scss you will have the rtl css files ready to use. 
The RTL files will override the original files.