That's all you can do. OpenOffice developers have reversed engineered the .xls document structure so that you can open .xls documents and even save documents as .xls. They have not been able to duplicate the .xls password setting technique.
M$ Office cannot even open an .ods document. When you are crossing from one structure to another, you cannot expect exact duplication.
BTW, anecdotal evidence is that password setting in OO is hazardous. There has been a significant number of reports of data being lost when an encrypted document becomes corrupted.
OpenOffice 3.3 on PCLinuxOS. There are 3 kinds of people: those who can count and those who can't.