The conclusions of the European Food Safety Authority (EFSA) following the peer review of the initial risk assessments carried out by the competent authority of the rapporteur Member State Belgium for the pesticide active substance COS-OGA are reported. The context of the peer review was that required by Regulation (EC) No 1107/2009 of the European Parliament and of the Council. The conclusions were reached on the basis of the evaluation of the representative use of COS-OGA as an elicitor to control powdery mildew (Sphaerotheca fuliginea) on cucurbits grown under glasshouses. An application for inclusion of COS-OGA in Annex IV of Regulation (EC) No 396/2005 was assessed. The reliable endpoints concluded as being appropriate for use in regulatory risk assessment derived from the available studies and literature in the dossier peer reviewed, are presented. Missing information identified as being required by the regulatory framework is listed.