It's pretty rare for a single application to lock up Linux to the point where your only option is a hard reboot. Before doing a hard reboot, there are other things you can try.
1. Right-click on the OpenOffice button on the bottom bar. It should bring up a menu. One of the options should be "close." If you select "close" it should either immediately close OpenOffice, or (after a few seconds) show a dialog that says OpenOffice is not responding and giving you the option to force it to quit.
2. See if you can start the Terminal application, which will give you access to the shell (Applications > Accessories > Terminal). If you are able to start this application, then you can force OpenOffice to quit using ps, then grep, then kill (not going into that level of detail here).
3. Control-Alt-F1 gives you another way to get to the shell, where you can again force OpenOffice to quit.
However, my suspicion is that OpenOffice is not really freezing. If it's really an old computer, it might just be VERY slow. Have you given OpenOffice a long time to start up, just in case that is the problem?