Easy access to Umlauts on MacOS X using KeyRemap4MacBook

private.xml

<?xml version="1.0"?>
<root>
  <item>
    <name>Easy access for Umlauts</name>
    <identifier>private.easy_umlaut_access</identifier>
    <autogen>
      --KeyToKey--
      KeyCode::A, ModifierFlag::OPTION_L | ModifierFlag::SHIFT_L,
      KeyCode::U, ModifierFlag::OPTION_L, KeyCode::A, ModifierFlag::SHIFT_L
    </autogen>
    <autogen>
      --KeyToKey--
      KeyCode::A, ModifierFlag::OPTION_L,
      KeyCode::U, ModifierFlag::OPTION_L, KeyCode::A
    </autogen>
    <autogen>
      --KeyToKey--
      KeyCode::O, ModifierFlag::OPTION_L | ModifierFlag::SHIFT_L,
      KeyCode::U, ModifierFlag::OPTION_L, KeyCode::O, ModifierFlag::SHIFT_L
    </autogen>
     <autogen>
      --KeyToKey--
      KeyCode::O, ModifierFlag::OPTION_L,
      KeyCode::U, ModifierFlag::OPTION_L, KeyCode::O
    </autogen>
    <autogen>
      --KeyToKey--
      KeyCode::U, ModifierFlag::OPTION_L | ModifierFlag::SHIFT_L,
      KeyCode::U, ModifierFlag::OPTION_L, KeyCode::U, ModifierFlag::SHIFT_L
    </autogen>
     <autogen>
      --KeyToKey--
      KeyCode::U, ModifierFlag::OPTION_L,
      KeyCode::U, ModifierFlag::OPTION_L, KeyCode::U
    </autogen>
  </item>
</root>

if you want to use SHIFT_R and OPTION_R also : https://gist.github.com/maxstauss/5090029#file-umlaut_access-xml