Like I said on my last post this is something that torment users all the time. Either because they uninstall OpenOffice, or update it, and something goes not exactly as planned and they loose of of these associations.
Bare in mind this really doesn’t depend on OpenOffice, but is handled by your operating system and could happen with any program, whenever is video or documents. So for that reason is usually on those places where you need to find answers. Here is one for Microsoft and one for Apple, and another for Gnome, Unity and KDE.
That said here is the general explanation in a nutshell. You have 6 different filetypes (for the most part) all ending in the following manner: docx, odt, pptx, odp, xlsx, ods or doc, ppt, xls. These files most be associated wtih an office suite that can run it. You might have more than one office suite in your computer, so you will need to tell the computer which application should be the default.
Fortunately most modern environments give you a context menu (the one you see when you right click) and see more options, including opening the document with alternative programs. This means that you can have a default application and also a secondary and other options. These menu however is managed by the file browser located in your sistem, so is logical that the configuration for such file management is either on the settings of your file manager (File explorer on windows, Finder for OSX, Nautilus and Dolphin for Gnome and KDE). So you will see a list of file types (you will be surprised how many they are) and their application right next to it. So here are some screenshot to get you familiarized with how it looks like:
Like many things in a desktop environment there are many ways to do the same thing, and this is also the case for file association edition. Just hope this guide clears how file association works and what to do to configure it.