Don't bother with the .xls file, it won't work on openoffice as posted because of differences in syntax. The .ods file only contains modifications to correct for the syntax differences between openoffice and excel. If you want to look at the .xls file contents, you could probably just re-save it under a new name and then load, look at, and modify it. If it's really an OS file permissions issue (not just openoffice preventing you from losing data by editing something in .xls that openoffice doesn't deal with correctly), then just right click on the file in the file browser (Nautilus by default) and then click on Properties, then the Permissions tab in the window that opens and give it the read/write/execute permissions you wish it to have. You may need to enter your root password to change permissions.
OK, I think I stay at .ODS file. Thank You for an advice!