1) Go to Tools > Extension Manager
2) Delete all of the language extensions except "American English Spell Checking 1.0" and "English spelling and hyphenation dictionaries and thesaurus 2010.03.16"
3) Close OpenOffice
4) Check to be sure that it's not still open in your task manager (bottom right of screen). If your Quickstarter is enabled, this must be closed as well. Right click on the OpenOffice icon and click "Exit Quickstarter" to completely close OO.
5) Restart OpenOffice. Your F7, Ctrl+F7, right click word to get synonyms and Tools>Language>Thesaurus features will all be there, working properly.
This is as simple as it gets. Good luck