Well, I don't use Open Office much, in fact I downloaded it just to play around and try to reproduce the issue.
If you open a new Calc (Spreadsheet), it is read/write. To change it to read, you would have to go to Tools --> Options --> Security and then change it to read only. From what I see, this cannot be changed back.
Of course, the next document should open as read/write and, therefore, you could upload that one.
The ONLY thing I can think of if you are getting every one to open as read only would be if you had made one read only and then saved it as an .stc file or template.
