Introduce --oss flag to use gpt-oss models (#1848)

This adds support for easily running Codex backed by a local Ollama
instance running our new open source models. See
https://github.com/openai/gpt-oss for details.

If you pass in `--oss` you'll be prompted to install/launch ollama, and
it will automatically download the 20b model and attempt to use it.

We'll likely want to expand this with some options later to make the
experience smoother for users who can't run the 20b or want to run the
120b.

Co-authored-by: Michael Bolin <mbolin@openai.com>

README.md

@@ -18,6 +18,7 @@ This is the home of the **Codex CLI**, which is a coding agent from OpenAI that
 - [Quickstart](#quickstart)
   - [OpenAI API Users](#openai-api-users)
   - [OpenAI Plus/Pro Users](#openai-pluspro-users)
+  - [Using OpenAI Open Source Models](#using-open-source-models)
 - [Why Codex?](#why-codex)
 - [Security model & permissions](#security-model--permissions)
   - [Platform sandboxing details](#platform-sandboxing-details)
@@ -186,6 +187,41 @@ they'll be committed to your working directory.
 
 ---
 
+## Using Open Source Models
+
+Codex can run fully locally against an OpenAI‑compatible OSS host (like Ollama) using the `--oss` flag:
+
+- Interactive UI:
+  - codex --oss
+- Non‑interactive (programmatic) mode:
+  - echo "Refactor utils" | codex exec --oss
+
+Model selection when using `--oss`:
+
+- If you omit `-m/--model`, Codex defaults to -m gpt-oss:20b and will verify it exists locally (downloading if needed).
+- To pick a different size, pass one of:
+  - -m "gpt-oss:20b"
+  - -m "gpt-oss:120b"
+
+Point Codex at your own OSS host:
+
+- By default, `--oss` talks to http://localhost:11434/v1.
+- To use a different host, set one of these environment variables before running Codex:
+  - CODEX_OSS_BASE_URL, for example:
+    - CODEX_OSS_BASE_URL="http://my-ollama.example.com:11434/v1" codex --oss -m gpt-oss:20b
+  - or CODEX_OSS_PORT (when the host is localhost):
+    - CODEX_OSS_PORT=11434 codex --oss
+
+Advanced: you can persist this in your config instead of environment variables by overriding the built‑in `oss` provider in `~/.codex/config.toml`:
+
+```toml
+[model_providers.oss]
+name = "Open Source"
+base_url = "http://my-ollama.example.com:11434/v1"
+```
+
+---
+
 ## Why Codex?
 
 Codex CLI is built for developers who already **live in the terminal** and want