Wireless sensor networks (WSN) are inherently multi - constrained. They need to preserve energy while offering reliable and timely data reporting for a non-negligible number of scenarios. This is particularly true when a node should decide which forwarder has to be chosen for routing a packet. Nevertheless, solving multi-constrained routing problems is NP-complete. Most approaches involve transforming the problem into a single constrained problem using a cost function, although this may lead to suboptimal solutions. Some other solutions are based on heuristics. However, their high implementation complexity prevents their online use. Hence, they are not suitable for highly dynamic WSNs. In this paper we make use of Operator Calculus (OC) methods on graphs to solve path selection in the presence of multiple constraints. OC has lower resolution time complexity compared to other techniques. Based on OC, we develop a distributed algorithm for path selection in a graph. We develop a new routing protocol which makes use of this algorithm: the Operator Calculus based Routing Protocol (OCRP). In OCRP, a node selects the set of eligible next hops based on the given constraints and the distance to the destination. It then sends the packet to all eligible next hops. The protocol is implemented in Contiki OS and emulated for TelosB motes using Cooja. We compare its performance against tree and directional flooding routing and show the advantages of our technique.