The reason OSI didn't approve it is because CC0 does not grant you a patent license. So you if you use CC0 "licensed" code in your own projects, you are free to do anything with such code from a copyright perspective -- but the author of the CC0'ed code has explicitly not given you a patent license for that work.

As detailed on D.J. Bernstein's page at http://cr.yp.to/publicdomain.html, a lot of the fear, uncertainty, and doubt about public domain software can be traced back to Lawrence Rosen's influential 2002 Linux Journal article "Why the Public Domain Isn't a License".

(For those who may not know, Rosen is the former general counsel and secretary of OSI.)

However, that article was incorrect on a number of issues, as Bernstein points out. And indeed, Rosen has last year at long last recanted his previous views on public domain software:

  > I have already voted +1 to approve the CC0 public domain dedication
  > and fallback license as OSD compliant. I admit that I have argued for
  > years against the "public domain" as an open source license, but in
  > retrospect, considering the minimal risk to developers and users
  > relying on such software and the evident popularity of that "license",
  > I changed my mind. One can't stand in the way of a fire hose of free
  > public domain software, even if it doesn't come with a better FOSS
  > license that I trust more.

Source: http://projects.opensource.org/pipermail/license-review/2012...

The "fire hose" Rosen mentions can be witnessed in action at e.g. GitHub, with volumes of code now being put into the public domain on an everyday basis: https://github.com/search?q=%22public+domain%22&type=Code&s=...

This FSF link says it is fine http://www.gnu.org/licenses/license-list.html#CC0

The OSI links are here http://opensource.org/faq#cc-zero the issue is about the patent clause in the fallback.

See also https://creativecommons.org/weblog/entry/27081

Thank you!