Wireless actor and sensor networks (WSANs) are increasingly being deployed for accurate monitoring of physical environments subjected to critical conditions such as fire, leaking of toxic gases and explosions. The order in which events are received can determine the correct interpretation of what is occurring in the physical environment being monitored. Therefore, the ordering of events in WSANs can be a major requirement for critical security monitoring applications to avoid ambiguities, increasing monitoring accuracy. This paper presents OBC (Ordering by Confirmation), a novel protocol for event ordering in WSANs. This low latency and energy aware protocol maximizes the network lifetime by ordering events through a confirmation mechanism, independent of network topology. OBC was evaluated against existing solutions, showing that it can provide a larger network lifetime as well as save more energy and achieve lower latencies in situations where the network density, event rate and network dimension increase. The main goal of this protocol is to meet the needs of critical conditions monitoring applications, contributing to reduce death risks and patrimony losses.