I'm not as familiar with EO's, but as I understand it after reading Tom's original post, Obama can enact legislation that further makes the current gun laws stricter.
I'm thinking since there's already a high-cap ban in certain states like CA, he will attempt to expand it nationwide.
He may also make background checks mandatory for every firearm sale including private sales. This would be tough but if caught, they will institute some sort of punishment/fine/jailtime.
Another is by possibly taxing or raising prices on ammo and mags, and limiting ammo purchases i.e. 6boxes of ammo per person at a time.
This is all speculation of course. I'm no expert but I had a in-depth discussion with my brother-in-law who just finished law school and he gave me the finer points of an EO.