In the field,Wireless Sensor Networks face the harsh reality of radio physics, including asymmetric and long-range transient links. Classic routing schemes are hindered by these links, and therefore prune them from the network topology. In this paper, we investigate how to put these opportunistic wireless links to good use and actually provide higher quality of service. We propose QOR, short for QoS oriented Opportunistic Routing protocol for data collection in Wireless Sensor Networks. Unlike classic routing schemes, QOR takes advantage of opportunistic links to provide faster and more reliable transmissions. Our contribution is threefold. First, we propose a joint routing structure and addressing scheme that allows to identify a limited set of nodes than can become opportunistic relayers between a source sensor and the sink. Second, we define an original cascaded acknowledgement mechanism that brings reliable acknowledgment and replication-free forwarding to the opportunistic communication scheme. Finally, our performance evaluation assesses that QOR efficiently uses opportunistic links to provide reliable and replication-free data delivery.