In the Opus standard tool bar "Menu", you can use the option "save as default lister" (or similar). Then, in -> settings -> preferences -> Launching Opus -> Startup, you can set Opus to use that default lister.
Here you can find more details about how to work with layouts:
You could also look, if -> settings -> preferences -> folders -> folder behaviour -> enable folder content type detection is set. If so, try to uncheck that option.
[ul][li] Make Opus always open in thumbnails mode by default?[/li]
[li] Make Opus always open the way you last used it?[/li]
[li] Make a specific layout you can open for special occasions that uses thumbnails, without changing how it opens in other situations?[/li]
[li] Make Opus always open a particular folder in thumbnails mode?[/li]
[li] Something else?[/li][/ul]
They are all possible, but I'm not sure which is wanted here.