Can I not install LibreOffice

"I am running a server dependent on Openoffice."

LibreOffice is very similar to OpenOffice.org. Are you sure that LibreOffice will not do what you need? Please feel free to post about your needs, if you are at all unsure. If you're running Ubuntu 11.04 Natty Narwhal, it is recommended to use LibreOffice rather than OpenOffice.org, and there are very few, if any, instances where it is necessary to use OpenOffice.org instead.

With that said, if you are willing to manually maintain your installation of OpenOffice.org (i.e., manually install updates when they are released), you can install it manually by going to http://www.openoffice.org, clicking "I want to download OpenOffice.org", and following the instructions. Again, I emphasize that if you do that, OpenOffice.org will not be updated when you update the rest of your system's software in the Update Manager. If you decide to do that and need help or encounter problems, please feel free to post a reply. I recommend first completely uninstalling LibreOffice, which you can perhaps do most easily by running the Synaptic Package Manager and marking all the packages listed at https://launchpad.net/ubuntu/+source/libreoffice for complete removal (they're probably not *all* installed...so make sure you don't accidentally install the ones that aren't installed).

On the other hand, if you're running any previous version of Ubuntu (before 11.04) and you've installed LibreOffice but you want to go back to OpenOffice.org, then all you have to do is completely remove LibreOffice (as detailed above), disable the entries for the LibreOffice PPA in your Software Sources (you can do this from the Synaptic Package Manager by going to Settings > Repositories, clicking the Other Software tab, and finding all the PPA entries relating to LibreOffice or The Document Foundation and unchecking them), and then install OpenOffice.org.