plugin_remove_wp_open_sans.php

<?php

// Remove Open Sans that WP adds from frontend

if (!function_exists('remove_wp_open_sans')) :
  function remove_wp_open_sans() {
    wp_deregister_style( 'open-sans' );
    wp_register_style( 'open-sans', false );
  }
  add_action('wp_enqueue_scripts', 'remove_wp_open_sans');
  // Uncomment below to remove from admin
  // add_action('admin_enqueue_scripts', 'remove_wp_open_sans');
endif;

?>

Remove Open Sans from Wordpress 3.8 .md

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.

Credit to seventhsteel from http://wordpress.org/support/topic/turning-off-open-sans-for-the-38-dashboard