It might be better to save a document as a template first. Open a new document and change the font at the top, then you need to find out where your version of OpenOffice keeps your Templates. Go to Tools>Options then click on Paths, you will see the location where OO stores it's Templates, make a note of it. Go back to the document then File>Save As....
when the window opens type in a file name, I suggest Tahmoa, open the File filter drop down menu and select OpenDocument Text Template .ott then navigate the the folder OO stores it's Templates then click save. You may have to click Show hidden files, depending on your system.
Close OO and reopen it, click on File go down to Templates and Documents, in the window you'll see a folder called My Templates, you should find the Template for Tahoma in there, that should be your default document for the Tahoma font in the future.