Are you sure you selected GPT 5.4-xhigh as model, in Codex? Because this makes a huge difference, and with this setting in my experience Codex outperforms Opus for almost every coding/reasoning task. Opus is still better often times when there is to call a lot of tools, interact with servers to do operations and alike, but not always. But for low level coding, Codex with GPT 5.4-xhigh is really powerful.