Container Standards: The OCI

Ensuring Interoperability

An introduction to the Open Container Initiative (OCI) and its role in standardizing container formats and runtimes, enabling tools like Podman and Docker to work together.
Author

Chuck Nelson

Published

November 16, 2025

1 Purpose

In the early days of containers, different tools used different formats, making them incompatible. To solve this problem, the industry came together to create a set of open standards. This document introduces the Open Container Initiative (OCI), the organization that defines the rules for how containers are built, run, and shared, ensuring that the container ecosystem is open and interoperable.

2 What You’ll Accomplish

By the end of this reading, you will be able to:

  • Define the purpose of the Open Container Initiative (OCI).
  • Explain why standards are important for container technology.
  • List the three main specifications that make up the OCI standard.

This reading maps to the following program and course learning outcomes:

  • Course Learning Outcomes (CLOs):
    • 1. Identify hardware and basic network components: This topic covers the industry standards that govern modern software deployment and virtualization technologies.

This exercise develops the following skills, which align with the O*NET SOC Code 15-1232.00 for Computer User Support Specialists.

Learning Objective O*NET KSAs Technologies Used
Define the purpose of the OCI. Knowledge: Computers & Electronics
Skills: Reading Comprehension		 OCI
List the three main OCI specifications. Knowledge: Computers & Electronics
Abilities: Information Ordering		 Image-spec, Runtime-spec

3 What is the Open Container Initiative (OCI)?

The Open Container Initiative (OCI) is a project hosted by the Linux Foundation. Its goal is to create open, industry-wide standards around container formats and runtimes. Before the OCI, the container world was like the early days of video tapes, with competing formats like Betamax and VHS. The OCI created a single, open “VHS” standard that everyone could agree on.

This means you can be confident that a container image you build with one OCI-compliant tool can be run correctly by another OCI-compliant tool.

3.1 Why Standards Matter

  • Interoperability: You are not locked into a single vendor. You can build an image with Docker, push it to a registry, and have it run by Podman or another OCI-compliant runtime without any changes.
  • Stability: The standards provide a stable target for developers. Tool creators can focus on adding features on top of the standard, rather than reinventing the basics.
  • Ecosystem: A common standard allows a rich ecosystem of tools for security scanning, monitoring, and orchestration (like Kubernetes) to flourish, as they can all rely on the same underlying container format.

4 OCI-Compliant Tools

It’s important to understand that tools like Podman and Docker are not the standard itself; they are high-level tools that use the OCI standards. They are OCI-compliant clients and engines that provide a user-friendly way to build, manage, and run containers. The actual low-level work of running a container is handled by an OCI-compliant runtime like runc.

5 The Three Core OCI Specifications

The OCI maintains three key documents that define how containers work.

  1. The Runtime Specification (runtime-spec): This document defines how to run a container. It specifies how a container’s filesystem should be set up and what actions a runtime (like runc) must take to execute the container process with the correct isolation. It’s the instruction manual for “how to press play.”

  2. The Image Specification (image-spec): This document defines the format of a container image. It specifies that an image is a “bundle” composed of a JSON configuration file, a set of filesystem layers (the application and its dependencies), and a manifest that describes the contents. It’s the blueprint for the container.

  3. The Distribution Specification (distribution-spec): This document defines the protocol for sharing container images. It specifies the API that a container registry (like Docker Hub or Quay.io) must provide, allowing clients like Podman and Docker to push (upload) and pull (download) images.

Together, these three specifications create a complete, standardized lifecycle for containers, from building to sharing to running.

6 Reflect and Review

ImportantReflection: 3-2-1

Now that you have reviewed this document, take a moment to reflect on your learning in your Microsoft Teams Student Notebook:

  • 3 core OCI specifications.
  • 2 reasons why open standards are important for containers.
  • 1 question you still have about the role of Podman vs. the OCI.
TipCheck on Learning

Answer these questions in your notebook to solidify your understanding:

  1. What is the primary goal of the Open Container Initiative (OCI)?
  2. Are Docker and Podman the OCI standard? Explain your answer.
  3. Which OCI specification defines the format of a container image?
  4. You use a tool to build a container image and upload it to a registry. Which two OCI specifications have you just used?
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