What you can try though is:

Log in on another device where you might still be signed in (sometimes old tabs/history are cached there)
Check if any browser profile/local backup still exists on your old Windows user folder
Contact Opera support, but realistically they’ll likely confirm it’s not recoverable after overwrite

For future, it’s a good idea to export bookmarks or keep a local backup alongside sync, especially for large history/tabs.

1. Check if your old data still exists locally. Go to your previous user folder backup (if you have one) and look for Opera profile data, especially the “History” and “Sessions” files. If you formatted but kept a backup or used something like Windows.old, you might still recover it.
2. Try logging into sync from another device (phone, old laptop, etc.) that might still have the old data cached. If it hasn’t synced yet, you could export bookmarks or session info from there.
3. Contact Opera support immediately. In some cases, they may have limited server-side snapshots, but timing is critical.
4. Avoid further syncing actions for now, as they can overwrite any remaining traces.
   For the future, it’s safer to keep periodic manual backups of your Opera profile folder instead of relying only on sync, since sync is designed for mirroring—not version recovery.