PS: OpenOffice can save in older Microsoft Office formats (such as .doc and .xls) but not in newer ones, such as .docx and .xlsx. If you want an office suite that can save into the more recent formats (not that it actually matters that much), use LibreOffice instead:
https://www.libreoffice.org
To set LibreOffice to automatically save your files into the latest Microsoft Office formats, go to Tools > Options. Double-click 'Load/Save' and then click 'General'.
With 'Document type' showing 'Text document', change 'Always save as' to 'Word 2007-365'.
Then change 'Document type' to 'Spreadsheet' and select 'Excel 2007-365' as the save type.
Next, select 'Presentation', together with 'PowerPoint 2007-365'.
Click 'Apply' and then 'OK'.
Then LibreOffice will save files in exactly the same way that Microsoft Office does.