oatmeal is a complex carb not a simple carb. When use before you go to the gym and have complex carbs in your system you won't be feeling them right away, in fact you might not even feel them until your workout is done. Where as simple carbs hit you with in 30 minutes or less.
I eat 1.5 hours before I workout, anything less and it slows me down.
Instant oatmeal (flavoured) has a shitload of sugar in it, hence it is a decent aft workout carb source. You get both complex and simple carbs which are necessary aft workout.