If you mean the old list view of your open tabs, I’d check the tab screen itself first. Opera has changed the Android tab interface a few times, and the options can depend on the version and layout you’re using. Opera’s current help only documents the visual tab view, so if you’re not seeing the list option anymore, it may have been removed or moved in your version.
I’d make sure Opera is updated, then open the Tabs screen and look for a small layout/view button near the bottom or in the three-dot menu. If it’s simply not there, tell us your Opera version and Android version. That’ll make it easier to tell whether you’re missing a setting or Opera changed the feature.