DE in most defenses are taught to read the block of the OT or whatever OL they are shaded outside. If the OT blocks down or onto a LB the DE is taught to close down the line of scrimmage. If the QB makes the correct read he should keep the ball every single time. There are variations (blitzes, twists, etc...) in every defense, but in the base defense the DE should be closing down the LOS and therefore showing the QB the "keep" read.
I have coached against option teams in the past (both spread and the more traditional triple) to varying degrees of success. When breaking down the film of the opponent, my coaching staff and I would determine which player was the biggest threat and we didn't want carrying the ball, whether it was the QB, FB, or HB. For only that week we would change our defensive keys to ensure the weakest (or who we thought was the weakest) player would be carrying the ball more frequently.
This strategy worked on multiple occasions, however in some games the other coaches would see what we were doing and adjust their play calling. For example if our DE was coming straight up-field because we wanted the QB to give the DE is very susceptible to a trap or kick-out block.
The bottom line is that the read option is fairly easy to understand, but the good teams have other plays in their playbook that they can easily go to if/when a team sells out to stop the option.
