With OpenOffice (and the majority of Mac apps), uninstalling is as easy as installing. Just as you copied the application to the Applications folder, just drag it to the Trash and empty it. Simple as that.
If the application came with an installer (like MS Office, for example), then it's best to use the uninstaller that came with it. It will either be a "remove' option in the Installer app used to install the application, or there will be a separate uninstaller for you to run in the same folder as the app you want to get rid of.
• Apple iMac G5 17" (2 GHz G5) - Mac OS X 10.5.8/Ubuntu 10.04
• Asus Eee PC 901 (1.6 GHz Atom N270) - Fedora 13
• Apple Macintosh Quadra 650 (33 MHz MC68040) - Mac OS 8.1
• "JHVH-1" (2 GHz AMD Athlon XP 2400+) - Slackware 13.1