A snaggletoothed bulldog of a fire captain told me that CPR revives 5% of people who've gone into cardiac arrest, and that the main value of CPR was to keep the cells oxygenated until a defibrillator becomes available.
I dunno if that percentage is right cuz I had CPR done on me after my heart stopped due to heroin and vodka. The CPR alone brought me back. Maybe I'm a 5 percenter.
I don't know exact percentages either, but it entirely depends on the cause for the arrest. In most adults it will be due to a problem with their heart - myocardial infarction, for example, or an abnormal rhythm. CPR forces oxygen into your lungs and blood around the body, and will keep tissues oxygenated, but you will need something doing about the primary heart problem as well.
Defibrillation is for when your heart goes into a specific, chaotic rhythm (fibrillation). The idea is if you apply a shock, you temporarily stop the heart and hope it restarts in a normal rhythm. It will not work for people who are "flatlining" and who's hearts have stopped completely, whatever medical shows may show you! Myocardial infarction is a common cause for fibrillation, but there are many.
Sorry, going slightly off track here. My point is that cardiac causes of cardiac arrest have a low recovery rate, essentially because the heart itself is damaged. In an OD situation, a respiratory arrest occurs. Cardiac arrest will follow shortly if nothing is done, as the heart muscle is deprived of oxygen. In this situation, the problem is not with the heart itself, and chances are a lot better for making a recovery following CPR and appropriate treatment. This depends on the amount of time the body is starved for oxygen, of course, so early and efficient CPR is essential.