In recent years, it has been a relatively common practice for the Polish antitrust authority – the Office of Competition and Consumer Protection (the OCCP) – to issue conditional decisions in merger control proceedings. Actually, this is not a practice known only to the Polish antitrust authority, as such decisions are also issued by the European Commission. Consents granted following this procedure allow participants to a planned concentration (also referred to as merger) to proceed, but under certain conditions – behavioural or structural. Several important issues arise especially with regard to the latter type of conditions, particularly in terms of the permissible degree of interference in the sphere of private legal rights and obligations of one or both participants in the concentration, and the consequences of failure to comply with the condition under which the concentration has been approved. These issues are the subject of this paper, which ends with conclusions and implications regarding the judicial practice of the Polish antitrust authority in the area in question.