Some CC licenses like CC-BY-SA and CC-BY can be compatible with free software licenses. It's often easier to just license the resources of a program the same as the code, so there are no conflicts.
Excuse my ignorance, but can OS licenses be applied to other than code (content, text, images, ... etc) ?
I mean, really ignore the impact of licensing content with OS license, any references?
Of course. The very last paragraph on this page explains that these licenses can be used for any kind of data.