The paper provides an analysis of the Organization of Security and Cooperation in Europe’s (OSCE) Special Monitoring Mission (SMM) to Ukraine within the multilateral attempt to manage or resolve the conflict in the Donets’ Basin (Donbas). It aims to tentatively assess the SMM’s influence on the de-escalation of the conflict during the Presidency of Petro Poroshenko, as a relatively closed political period. We explore the Mission’s mandate, various evaluations of its operations and effectiveness, as well as the conflict participants’ interaction with the Mission. The nature and dimension of the conflict have posed an, in its dimension, novel, yet, in its nature, not untypical challenge to the consensus-based OSCE. Despite the challenges of the Mission’s specific scope and the limits placed on it by both Russian sabotage and a lack of resources, the SMM nevertheless managed, in 2014-2019, to contribute to de-escalation in the Donbas. Determinants of the net positive contribution of the SMM include its continued and large presence on the spot, the improvement of its reports on the situation in the conflict zone, as well as the development of more sophisticated monitoring methodologies and technologies over time.