We present Omid – a transaction processing service that powers web-scale production systems at Yahoo. Omid provides ACID transaction semantics on top of tradi- tional key-value storage; its implementation over Apache HBase is open sourced as part of Apache Incubator. Omid can serve hundreds of thousands of transactions per second on standard mid-range hardware, while in- curring minimal impact on the speed of data access in the underlying key-value store. Additionally, as expected from always-on production services, Omid is highly available.