From the OpenOffice.org documentation wiki, How Tos - Edit Keyboard Configuration File suggests making shortcuts containing the Alt key isn't available from the interface but can be done by editing a config file. The bug report linked to from that page says Alt shortcuts are available in OpenOffice.org 3.2.
However, at the bottom of the How To page, there's a section for Verson 3.1 on (Ubuntu) Linux that says
On Ubuntu Linux, it seems that the functionality of the Alt-Gr key can't be modified by OpenOffice. Therefore D_MOD1_MOD2 (=ALT-Gr) won't work. However, D_MOD2 for the standard ALT key (on the left side of the long space key) works well as described in the section on OOo 3.1 above.
Earlier in the page it's explained that MOD1 is Ctrl and MOD2 is Alt, so this seems to say that OpenOffice.org can't control Ctrl+Alt combinations in Ubuntu Linux.