The reason you might not be able to remove the Open Sans font that Wordpress >= 3.8 adds to the frontend is that quite a few WP styles and scripts list 'open-sans' as a dependancy when being registered and enqueued. When you remove the 'open-sans' style the other plugins dependant on it will not load. So you just need to deregister WP's open sans style and register your own, with a
false value for the
src like below.