I'm thinking about the armbar and omoplata from the guard as a nice inside/outside attack.
With the armbar from the guard, you want to bring the targeted arm in, with the omoplata, you want the arm of the targeted shoulder out.
I also think of the armbar as an inside-to-outside attack. You're pivoting your body into the guy to begin the attack. The attack is finished on the outside, with your outside leg coming down across the neck to immobilize the guy's body while you hyperextend the arm.
On the other hand, the omoplata is an outside-to-inside attack. You swing your body outside the arm that's on the same side as the targeted shoulder. But the attack is finished on the inside, as you bring the arm of the targeted shoulder backwards across his back ("put his wrist on his ear").
