OpenOrg is somewhat particular about filename extensions. When I created a test file with no extension (routine on Macs), OO would only recognize the file if the File Type in the Open File dialog was set to *.*. And then it opened the file in Writer. However, if the extension was .csv, it opened in the spreadsheet mode. For the .txt extension to open as a spreadsheet, select File type... Text CSV (*.csv; *.txt). Then the file will open as a spreadsheet. There is another dialog that allows selection of the field separator (tab, comma, etc.).
The OO help got me to this point, but it is not entirely clear. I had to try several things.
Lynn