After changing your Ubuntu system icon theme you may find that OpenOffice’s toolbar icons have disappeared. There’s an easy fix for this.
Launch OpenOffice and open Tool->Options. Select View from the list on the right of the options window. Open the icon theme drop down box and select Human. The icons will be restored once you close the options window.
The problem seems to be OpenOffice changing its icon theme to match the system setting, when the system setting does not work with OpenOffice.