To the extent that either organization possesses either the moral authority or the political strength to compel a change in how companies do business, it is very limited. If forced to choose, however, the United Nations carries greater weight, as evidenced by its sanction, and the moral weight that carried, in institutionalizing the Kimberley Process for preventing so-called "blood diamonds" from entering the global marketplace. While the Organization for Economic Cooperation and Development has 34 members, although many more countries participate in its meetings and forums, it pales in comparison to the United Nations. The United Nations General Assembly has 193 members, plus numerous nongovernmental associations that work with or through the myriad U.N. agencies.
In addition to its far greater membership, the symbolism of United Nations resolutions carries far greater weight than do guidelines issued by the OECD. This is not meant to denigrate the OECD; on the contrary, some of the world's largest economies are represented in the OECD. It's European origins and focus, however, limit its symbolic reach. That the advanced countries that are members agreed to the guidelines on how to conduct business overseas and at home should not be underappreciated, and the fact that eight additional countries signed onto the OECD guidelines, including Latin America's economic giant, Brazil, increases the importance of these measures. There is no question, however, that the broader reach of the United Nations lends greater authority to its Global Compact.
Because the guidelines are similar in intent and scope, the preponderance of importance has to reside with the United Nations Global Compact.