The fact that cyber attacks are getting increasingly sophisticated and performed at machine speed motivated the development of OpenC2. This paper presents Open Command and Control (OpenC2), a suite of specifications that enable command and control of cyber defense systems and components at machine speed and in a manner that is agnostic of the underlying technologies utilized or of any other aspects of particular implementations. OpenC2 provides the means to introduce standardized interfaces to cyber defense systems, enabling interoperability and allowing seamless integration, communication, and operation between decoupled blocks that perform cyber defense functions. The suit of specifications includes a semantic language that enables machine-to-machine communication for purposes of command and control of cyber defense components, actuator profiles that specify the subset of the OpenC2 language and may extend it in the context of specific cyber defense functions, and transfer specifications that utilize existing protocols and standards to implement OpenC2 in particular environments. Fundamentally, OpenC2 addresses the acting part of the Integrated Adaptive Cyber Defense (IACD) framework and is designed to be technology agnostic, concise, abstract, and extensible. Ultimately, OpenC2 is a building block for enabling coordinated defense in cyber-relevant time, shifting traditional monolithic cyber response approaches to more granular, flexible, and adaptive.