This whitepaper introduces Overledger, a proposed technology aimed at resolving the issue of single-ledger dependency in distributed ledger technologies (DLTs). The authors argue that the current DLT landscape lacks seamless interoperability, hindering widespread adoption and the development of multi-ledger decentralised applications. Overledger seeks to act as a new blockchain operating system by creating a messaging layer and a filtering and ordering layer that sit above existing blockchains. This approach intends to decouple application logic from the underlying ledgers, enabling communication, migration, and data exchange across different DLTs. The paper contrasts Overledger with existing interoperability projects, details its architectural layers and components like the Blockchain Programming Interface (BPI) and Verification Blocks, and discusses concepts like cross-ledger transactions and two-phase commits. Finally, it explores potential use cases, including the Command Query Responsibility Segregation (CQRS) pattern, to illustrate the applications of Overledger.