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.