Out-of-hospital cardiac arrest (OHCA) is one of the leading causes of mortality in the industrialized world. Bystander CPR before arrival of the Emergency Medical Service (EMS) is associated with an increased chance of survival. During the last decade, the best form of bystander CPR has been debated. Chest Compression Only CPR (CO-CPR) has been advocated as a preferable method in situations where the bystander has no previous knowledge in CPR, both because its believed to be equally efficient but also a simplified form of CPR that could lead to a higher incidence of bystander-CPR. In an initiative to increase CPR rates the American Heart Association has launched public campaigns such as the hands-only CPR promoting CO-CPR as an option to S-CPR for adult non-asphyxic cardiac arrest. In the 2015 updates of the European resuscitation council guidelines it states that the confidence in the equivalence between the two methods is not sufficient to change current practice. Whether CO-CPR leads to a survival rate no worse than, equally effective, or even superior to standard CPR in situations where the bystander has previous CPR training however remains unclear. This clinical question remains unanswered while millions of people are trained in CPR worldwide each year. The overall purpose with this research project is to investigate whether instructions to perform a simplified form of CPR consisting of compressions only (CO-CPR) to bystanders with prior CPR-training is non-inferior, or better than, standard CPR (S- CPR) in witnessed Out-of-Hospital Cardiac Arrest (OHCA).