Tab auto-complete searches the directories in your $PATH variable.
Sometimes when you auto-complete it's not possible to uniquely identify which file you want, so it completes as much as it can. In your case there are probably two or more files which begin with "openoffice".
If you press tab again, it will display the names of all files in your path which begin with openoffice, e.g. openoffice-bin, openoffice2.0, etc...
Note that on my system open office is launched by typing ooffice
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