The paper proposes a MAC layer protocol called QoMOR (QoS-aware MAC protocol using optimal retransmission) that is designed to provide QoS in a decentralized network with multiple sensor (source) nodes that do not have the capability to receive acknowledgements from the sink node. A minimum data delivery probability between the sensor and sink nodes is achieved by allowing each source node to transmit each new packet an optimal number of times in every interval of time. The paper first discusses the single QoS class case and derives the maximum achievable frame delivery probability and then extends the concepts to the multiple QoS class case. It also addresses a few design optimization criteria. Simulation and numerical analysis results show that QoMOR can effectively provide QoS guarantees under distributed control without using conventional ARQ-based schemes.