Opus is an Anthropic/Claude model, not an OAI/Codex model. I’m not sure if you can actually use an Anth model on Codex these days, but my experience to date indicates that performance tends to be best when you match the model to the harness it’s intended for.

You might also consider using the mid tier model (opus, Terra) to generate a detailed plan and then use a lower tier model (sonnet, Luna) to actually implement each task in the plan.