The high level summary is that once Oracle acquired Sun some folks became concerned about Oracle since even though OpenOffice was open sourced Sun and now Oracle still own the trademark on "openoffice.org". Right now the two project are probably pretty close in functionality and the file formats are standardized. In the future functionality of the project will likely diverge somewhat but probably not a lot.

http://news.ycombinator.com/item?id=1735052

http://www.documentfoundation.org/faq/