If you want to open a single instance of Text document with OpenOffice, right click and open with OpenOffice.
If you want to have OpenOffice open just that file with OpenOffice all the time, then Get info and open with OpenOffice and close. Double click the file.
If you want to have OpenOffice open all Text documents with that same format (like .txt, .rtf, .doc etc.) extension then click on a file, then Get Info then Open with: OpenOffice and then click Change All before closing the Get info window.
BTW, LibreOffice is the new fork of OpenOffice since Oracle bought Sun and is really not paying much attention to it.
So LibreOffice is what you should be using now and it will open your OpenOffice files just the same.
As far as opening all Text documents, the Info Window does not allow me to make the choice of Open office. When I click on Open Office, text edit is what comes up. It won't allow mw to change it.
I am running Mountain Lion 10.8.2 and am haing trouble sownloading LibreOffice.
I get the following error message:
"LibreOffice.app" can't be opened because it is from an unidentified developer.
Your security preferences allow installation of only apps from the MAc App Store and identified developers.
Safari downloaded this file today at 8:18AM from www.libreoffice.org.
I appreciate you help.