Hi there,
I had the same issue. You can download an older version of Opera and install that. I used this link, which is a stable release from July this year.
https://get.geo.opera.com/pub/opera/desktop/120.0.5543.128/mac/Opera_120.0.5543.128_Setup.dmg
Then you need to run a command in Terminal to prevent autoupdates. Please seek advice elsewhere to learn how to do that.
But be aware that this stops the machine you have this on, to synchronize with another device running more recent Opera on Mac OS 11.8