yum install obsolete openoffice instead of libreoffice
I found that, when installing openoffice, adding exclude=libreoffice* to your /etc/yum.conf helped with sudo yum update, and I am sure that it would help with your problem too. It tells yum to ignore libreoffice* packages, therefore yum does not even know that there is a package that obsoletes openoffice.
sudo yum --setopt=obsoletes=0 install openoffice*
Or you can put obsoletes=0 to /etc/yum.conf
Both ways worked for me on Oracle Linux 6.7.