Start a terminal and launch Opera via the following command:
open -a /Applications/Opera\ Beta.app --args --with-feature:activation-order-tab-cycling=true
To make that permanent, create a small launcher application. To do this start AppleScript Editor
and then choose "File → New (⌘N)" and enter this command:
do shell script "open -a /Applications/Opera\\ Beta.app --args --with-feature:activation-order-tab-cycling=true"
Now "File → Save (⌘S)" as "File format: Application" in a suitable location (/Applications perhaps?). Finally add this launcher application to your Dock and use it to start Opera.
P.S. If you want to start Opera developer this way adjust the script accordingly. It will also be possible to do the same for Opera stable, once version 25 is released.
It didn't used to work but following changes we made for tab preview it does. And yes, I will open a bug to make the flag accessible via the flags ui on Mac.
In the mean time you can be one of the only people on a Mac who can use this form of tab cycling. ;)