In the Opera portable installation folder (which is just a Standalone (USB) installation), you can rename "opera_autoupdate.exe" to "opera_autoupdate.off". It's in a folder with the Opera version for its name. For example, "45.0.2552.812". There may be multiple version folders. You want the one with the highest version. Or, just rename "opera_autoupdate.exe" in all of them if you're unsure.
Then, whenver you do want to update, change it back from "opera_autoupdate.off" to "opera_autoupdate.exe", start Opera and goto "Menu -> About Opera". After the upgrade, you can rename the "opera_autoupdate.exe" in the new version folder.
Would be nice to be able to control this with an advanced setting in Opera though.
And appears the worning that flash player is expired and I must to update it too.
If Opera itself warns you about flash, I'm not sure how to disable that. For the Adobe Update service on each windows computer you use Opera Portable on, see https://www.technipages.com/disable-prevent-flash-player-update-prompts, which might work if you have admin access.