By default OO saves in a different format. To change these open Writer and go to Tools>Options. In the left hand pane click the + sign next to Load and then select General. In the right pane alter the bottom right hand drop down box to Microsoft Word/2000/XP. Then select Spreadsheet in the left drop box and as before for the right drop box. Hope that makes sense.
You will have to open your current existing Writer documents and do a "Save As" exercise to allow Word to open them. In future all documents you create in OO will be saved with the MS default file extensions.