OCP introduces an innovative relayer mechanism that seamlessly links applications to a modular, permissionless custody protocol. Functioning akin to a mesh network, OCP's protocol empowers end-users to utilize a diverse array of key management protocols through what we term "Keychain Aggregation." By establishing a protocol designed to address Keychain Aggregation, OCP not only simplifies developers' processes by abstracting risk and complexity but also consolidates and aggregates the economics associated with numerous bilateral commercial relationships. Through OCP, we introduce a novel custody primitive that embraces both modularity, facilitating connections with key management solutions, and decentralization, enabling users to access key generation and transaction signing services. We are particularly enthusiastic about pioneering this approach, observing the struggles of existing security and custody solutions to adapt to the growing proliferation of chains. Furthermore, with the imminent tokenization of Real World Assets (RWAs), potentially funneling trillions of dollars into the blockchain ecosystem, the need for versatile and modular solutions becomes increasingly evident.