Start with the above link and follow the series of articles. They detail one of the most common and successful systems used to defend the flexbone option offense.
This is basically how Ohio State defended Navy in 2014 with the DL and Linebackers. Ash employed a different scheme rotation in the secondary than detailed in the article. Ash used more of a man-match concept to rotate with the orbit motion. To contrast, Iowa used more of a hard or traditional Cover 2 scheme.
Regardless what scheme you use to try and stop (really, slow down) an option team is you have to take away the dive first. The defensive line needs to literally try and smash down the offensive line into the interior gapes to force the QB to pull the ball.
Talking to Fickell after the 2014 Navy game, he said in hindsight if he had to play an option team again he would play 4 true defensive tackles and align them in heavy shades on the offensive line. You CANNOT have your defensive line running up field and "attacking" as this makes the read easier for the QB and to double down, allows the offensive line to climb to the linebackers easily.
At the 2nd Level, your linebackers need to read the offensive line for "cloudy/clear" and not be concerned with who has the football. A common practice method is to defend the option without a ball so that each player is reading for their appropriate fit and NOT looking for who has the ball. If a halfway decent option coach sees that your defensive has predetermined who has what phase of the option, they will take your rules and beat you with them. An example of this, if you have your OLB automatically scraping outside for the QB to defend the veer, expect midline follow and you have the wing leading up on your safety.
In the secondary, you need to have a rotation to account for the pitch phase, the 3rd phase of the option. This could be a hard corner in Cover 2, a rotating Cover 3, or a man concept like Ohio State used in 2014. They would actually have the backside safety play flat across and attack the pitch man.
The key position in the scheme is your Mike or Middle Linebacker. Typically, you see teams play him at 7 yards deep so that he is free to run to the football. This needs to be a dude. As he needs to be able to run from Dive-Quarterback-Pitch as the extra hat.
My purpose was to provide a brief overview of what we could potentially see Ohio State do defensively against Army. If you have any specific questions feel free to fire away.