For those who might be having edge case behavior, or oddity with start/restart of opencode not retaining last used model properly, or "changing to a different model name" it's likely because of incorrect recommended setup in the opencode.json config file documented in the plugin repo.
You might try the config below, where model id is the key in the models object rather than user-friendly name.
This solved the odd behavior for me with launching opencode and /models slash command - it also provides shorter model names and easier search for all models with "oauth".
