Is it safe to install LibreOffice alongside OpenOffice.org?
Solution 1:
Yes absolutely. Considering you are using ubuntu and openoffice.org is installed by default, the libreoffice installer will install under the /opt directory thus not interfering with Oo installation.
Solution 2:
If you're installing via the PPA then no, it will replace OpenOffice.org with Libre Office.
If you're installing via the tarballs on their website, then tinhed's answer is correct.