OpenOffice is developed by Sun Microsystems, which has been bought Oracle. Oracle is not known for its support of open source projects. Because of this many of the OpenOffice people have left Sun and have started the Document Foundation. They have forked OpenOffice and renamed it LibreOffice. Linux Mint currently uses OpenOffice as the default office suite. And it is my understanding that it will remain the default in Linux Mint 11. Currently it is not avaiable in the repositories. But below is a link to install it using a PPA.