This paper presents a causally consistent key-value store Orion, which uses a novel protocol for Read Only Transactions (ROT). Unlike most of the existing protocols, Orion uses only one round of communication in the best case, and not more than two rounds in the worst case. We provide a theoretical bound on its communication complexity and qualitatively compare it with recent ROT protocols. We also quantitatively compare Orion with state-of-the-art protocol CausalSpartanX and illustrate that Orion achieves up to 1.7× higher throughput and generates 10× fewer messages on widely-used YCSB workload. © 2020 Copyright held by the owner/author(s). Publication rights licensed to ACM.