Hi!

I'd like to replace the folder icon with some other icon from kde-look.
Eg, It' not about replacing some specific folder icon but _all_ icons.
So when browsing around with Dolphin all folders will have a different look than the regular Oxygen look.

Is it possible?

1) You can replace the icon files with your icon file. You can find them in this folder:
/usr/share/icons/oxygen
In the subfolders there are files in different sizes.

2) Another way is that you can copy /usr/share/icons/oxygen to
/usr/share/icons/<find_a_name>
and replace the icon files in this folder. The advantage is that after an update your icons are not overwritten by the package manager. But then you've got the hole oxygen theme twice and you won't get new icons which are added by an update if you use your own theme.
If you use this method change the file index.theme in the new folder to get another name for your theme. Then select the right icon theme in systemsettings.

Thanks for your answer!

I also see that I've a whole Oxygen folder in ~/.kde/share/icons
Can I mess with the icons in this folder?
There many sizes, which one is of interrest?

You need to change all sizes.

And any idea what the local folder "~/.kde/share/icons/Oxygen" is used for?

No idea... although that is probably user installed icons, so if you are installing for just one user, install there.