As far as I know, OOo Base can connect to the mdb file to the extent that it can open tables and queries. I think something else - forms, reqportes, VBA and native macros - would need to be rebuilt.
I don’t think OOo Calc can read or execute VBA: it has its own Basic dialect, which has variant syntax. In my opinion, this is not how I understand it, supports the concept of a class. Some work seems to be happening, however, see the OpenOffice Wiki for more information. Right now, I would say you should plan a thorough and complete rewrite.
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