I have been trying to do this for years. I want the modified column next to the name in ALL my listers. I change the current lister by dragging the modified column next to the name. I then click settings save as default lister and it says it did but whenever I open a new lister it still has the mod after size and type. I don't customize Opus much but this should be simple. 
Please see the Folder Formats FAQ.
If you need even more details than the above guide (you probably won't), then the more detailed guide which it links to has them:
[url]Folder Formats: Detailed Guide]
So, basically, your changes are getting overridden by other formats.
If you hover the mouse cursor over the lock symbol at the bottom right,
you can see, from which folder format your changes are coming.