What Is OpenAI? ChatGPT, Codex and the GPT Models Explained

OpenAI is the AI company that makes ChatGPT, the GPT family of models behind it, and Codex, its coding agent. It also offers image and video generation tools and a developer API that many other apps are built on. If you have used an AI chatbot, there is a good chance it was ChatGPT or something powered by an OpenAI model.

This guide explains what OpenAI makes, how the pieces fit together, and how to use its models in AI agents.

OpenAI in one paragraph

OpenAI builds large AI models and the products around them. Its best-known product is ChatGPT, a general-purpose assistant for writing, research, analysis, coding and much more. Underneath, ChatGPT runs on OpenAI’s GPT models, which developers can also access directly through an API. Around that core, OpenAI offers tools for images, video, voice and code.

ChatGPT: the assistant

ChatGPT is a chat app available on the web, on desktop and on mobile. You type or speak, and it answers, drafts, explains, summarizes or helps you think through a problem.

Over time it has grown well beyond plain chat. Depending on your plan, ChatGPT can:

• Search the web and cite sources.
• Read and analyze files you upload, such as PDFs and spreadsheets.
• Create and edit images from a description.
• Hold voice conversations.
• Remember preferences across chats, if you allow it.
• Run custom versions of ChatGPT set up for a particular task.

Features and plan limits change often, so check OpenAI’s own pages for the current details.

The GPT models

GPT is the name of OpenAI’s main family of language models. Like other large language models, they are trained on huge amounts of text and learn to predict and generate language, which lets them answer questions, write and reason through problems.

OpenAI offers several models at once rather than one. Some are larger and better at hard problems, some are smaller and faster, and some are tuned to spend more time reasoning before they answer. ChatGPT often picks a suitable model for you, while developers choose explicitly in the API.

OpenAI’s main models are closed: you use them through ChatGPT or the API, and the model files are not published. OpenAI has also released some open-weight models that people can download and run themselves, so its approach is not purely closed.

Codex: OpenAI’s coding agent

Codex is OpenAI’s coding agent. It reads your project, edits code across files and runs commands such as tests. You can use it with a ChatGPT account or an API key, which makes it an easy step for anyone already paying for ChatGPT.

Codex can work in your terminal on your own machine, and OpenAI also offers ways to hand off tasks and check back on the results later. Its command-line tool is open source.

For a fair comparison with the alternatives, read Claude Code vs Codex vs Gemini CLI. If coding agents are new to you, the beginner’s guide to AI coding agents covers the basics.

Image and video generation

OpenAI also makes generative tools for media:

• Images. You can describe a picture in ChatGPT and get an image back, then ask for changes in plain language.
• Video. Sora is OpenAI’s video generation tool, which creates short video clips from text descriptions.

These tools are useful for mockups, illustrations and creative experiments. As with any generated media, check usage terms before publishing and be careful with images of real people.

The OpenAI API

The OpenAI API lets developers build OpenAI models into their own products. Many apps, from writing tools to customer support systems, use it behind the scenes. Through the API, developers can:

• Send text (and often images or audio) and get responses back.
• Let the model call functions or tools defined by the developer.
• Generate embeddings, numeric representations of text used for search and recommendations.
• Convert speech to text and text to speech.

API usage is billed separately from a ChatGPT subscription, so it’s worth knowing which one a tool needs before you sign up.

How OpenAI compares with other AI companies

OpenAI is one of several major model makers. Anthropic makes Claude, Google DeepMind makes Gemini, Meta makes the open-weight Llama family, and Mistral AI makes both open-weight and commercial models.

OpenAI’s distinguishing traits are breadth and reach. ChatGPT bundles a wide range of features in one app, and the API sits underneath many AI-powered products. For a side-by-side view of all of them, see the big AI companies compared.

Using OpenAI models in AI agents

AI agents are systems that plan and carry out multi-step tasks, not just answer a single question. OpenAI models are a common choice for them. Some practical advice:

1. Match the model to the step. Use a stronger reasoning model for planning and tricky debugging, and a faster one for routine edits.
2. Give clear acceptance criteria. Tell the agent which tests must pass or what “done” looks like.
3. Check permission settings. Coding agents can run commands, so understand what Codex is allowed to do without asking before you give it more freedom.
4. Review before merging. Read the diff and run the tests, even when the agent says everything passed.
5. Get a second opinion. A reviewer built on a different model, such as Claude or Gemini, often catches mistakes the first model misses.

That last point is why many people mix providers. In BotQuarters, a team of AI coding agents on your Mac, a teammate can use Codex with your ChatGPT account while others use Claude Code, Gemini, OpenCode or a local model.

Common misconceptions

• “ChatGPT and GPT are the same thing.” GPT is the model family; ChatGPT is the app built on it.
• “OpenAI makes Claude.” Claude is made by Anthropic, a separate company.
• “A ChatGPT subscription covers the API.” They are usually billed separately, though some tools, such as Codex, can sign in with a ChatGPT account.
• “All OpenAI models are closed.” The main ones are, but OpenAI has also published some open-weight models.

FAQ

What does OpenAI make?

OpenAI makes ChatGPT, the GPT family of models, the Codex coding agent, image and video generation tools such as Sora, and a developer API that other products use to access its models.

Is ChatGPT the same as OpenAI?

No. OpenAI is the company, and ChatGPT is its assistant app. ChatGPT runs on OpenAI’s GPT models, which developers can also use directly through the API.

What is OpenAI Codex?

Codex is OpenAI’s coding agent. It can read a codebase, make changes across files and run tests, and you can use it with a ChatGPT account or an API key.

Is OpenAI open source?

Mostly not. Its main GPT models are closed and used through ChatGPT or the API. OpenAI has released some open-weight models and some open-source tools, including the Codex command-line tool.