Witnessed cases of OHCA with a non-asphyctic aetiology will be the principal study population. According to current guidelines an unresponsive patient with no or agonal breathing is treated as a suspected case of OHCA at the dispatch centre. These cases are eligible for screening for inclusion. If the case is witnessed, the bystander has previous knowledge in CPR and no exclusion criteria are present, the case can be included in the study. If included, it will be randomly assigned to either CO-CPR (intervention) or S-CPR (control).