just go to your Software Center and search for Open Office
It is not in 18.04's Software Centre, unless it has been added with one of the subsequent point releases - 18.10, 19.04, or Dean's 19.10.
Dean if this is the site you downloaded from
https://www.openoffice.org/download/
... the site reads what sort of Linux you are using and offers either rpm for Fedora &c or deb for Ubuntu, Debian &c - so you may have changed that by mistake.
Consider first what Alexzee has suggested with
Libre Office is a nice alternative to Open Office.
I've been using Libre Office for the last say 8 years and it runs great.
You will find it in Ubuntu if you go to Activities - Search - Libre Office (if it is there, you do not need to report back on that command I gave you).
Libre Office is the defacto standard for most Linux, but some use Open Office.
I began using Open Office with version 1.0 in May 2002, when MS Office was giving me the Tom Tits (rhymes with), and used it on Windows and later on LInux until 2014, at which time I embraced Libre Office. Libre Office was "forked" from Open Office just before Apache bought them around 2010-2011, so they are very similar, next to no learning curve.
See what you think, and if you choose to stick with Open Office, then first get the right download
Apache Open Office 4.1.7_Linux_x86-64_install-deb_en-US.tar.gz
and then I'll give you two (2) articles to follow either of, and they include how to get rid of Libre Office first.
Cheers, Friday in Australia, so
Enjoy your Linux and
Avagudweegend
Wizard