You just go to https://forums.opera.com/search?term=&in=titlesposts. You can get to it by clicking the magnifying glass icon at the top of the page and just hitting enter. There you can select the category you want to search in. You can also type in the user box to only search for a certain user.
For example, to search for all "Opera for Android" posts by you and sort them by last reply time in descending order, you could create a search for:
https://forums.opera.com/search?term=%s&in=titlesposts&by=kdsdata&categories[]=20&sortBy=teaser.timestamp&sortDirection=desc&showAs=posts
in "Menu -> settings -> browser -> search -> manage search engines" and set a keyword like 'os'. Then, in Opera's address field, you'd type 'os' then a space and press enter. Or, type os term.
Of course, you could just use the forum search page to do that. 🙂
