Tkinter has several built-in fonts — TkDefaultFont, TkTextFont, TkFixedFont, etc. These are all what are called “named fonts”. They are remarkably powerful — change one of these and all widgets that use them will change as well.
To change one of these fonts, get a handle to it and then use the configure method to change. For example, to change the size of TkDefaultFont to 48 you would do this:
default_font = tkFont.nametofont("TkDefaultFont")
default_font.configure(size=48)
That’s it. You don’t have to do anything else — everything that uses TkDefaultFont will instantly notice the change.
In your question you imply you want TkDefaultFont font to be used by everything. To do that you can use option_add as you’ve shown:
root.option_add("*Font", default_font)
Note, however, that option_add only affects widgets created after you’ve called option_add, so you need to do it before creating any other widgets.
Also note that you can give the font name to option_add if you don’t want to bother with getting the font instance first (ie: root.option_add("*Font", "TkDefaultFont")).