Edge computing is a paradigm that extends cloud computing with storage and processing capacity close to the edge of the network that can be materialized by using many fog nodes placed in multiple geographic locations. Fog nodes are likely to be vulnerable to tampering, so it is important to secure the functions they provide. A key building block of many distributed applications is an ordering service that keeps track of cause-effect dependencies among events and that allows events to be processed in an order that respects causality. In this paper we present the design and implementation of a secure event ordering service for fog nodes. Our service, named Omega, leverages the availability of a Trusted Execution Environment (TEE) based on Intel SGX technology to offer fog clients guarantees regarding the order in which events are applied and served, even when fog nodes are compromised. We have also built OmegaKV, a key-value store that uses Omega to offer causal consistency. Experimental results show that the ordering service can be secured without violating the latency constraints of time-sensitive edge a