CSS grid for custom theming in Omega?
Absolutely, one might argue that's one of the reasons to use Omega in the first place; as per the first two features listed on the project page:


Fully Responsive Grid layouts based on 960.gs standards.
12, 16, 24 Column Layouts built in.


Have a look in the CSS files with names like alpha-default-normal-12.css under omega/alpha/css/grid/alpha_default/* to discover what classes are available for you to use. 

Those files should make it fairly obvious what conventions Alpha/Omega use for naming those classes too, so you should be able to predict what you need to use easily.