With recent advances, the trend has shifted from battery-powered wireless sensor networks towards ones powered by ambient energy harvesters (WSN-HEAP). In such networks, operability of the node is dependent on the harvesting rate which is usually stochastic in nature. Therefore, it is necessary to devise routing protocols with energy management capabilities that consider variations in the availability of the environmental energy. In this paper, we design OR-AHaD, an Opportunistic Routing algorithm with Adaptive Harvesting-aware Duty Cycling. In the proposed algorithm, candidates are primarily prioritized by applying geographical zoning and later coordinated in a timer-based fashion by exchanging coordination messages. An energy management model is presented that uses the estimated harvesting rate in the near future to adjust the duty cycle of each node adaptively. Simulation results show that OR-AHaD exploits the available energy resources in an efficient way and increases goodput in comparison to other opportunistic routing protocols for WSN-HEAP.