The Osterhagen Key was a device that was held by UNIT by the time of the New Dalek Empire's invasion of Earth.
After Earth fell to the Daleks, General Sanchez of UNIT gave the key to Martha Jones along with the prototype of Project Indigo to escape. Former Prime Minister Harriet Jones told Martha Jones not to use the key under any circumstances . (DW: The Stolen Earth)
The name was posibiliy derived either "Eastern City" or "Fossil City", depending on spelling. Also, "Osterhagen" is an anagram of "Earth's Gone" or "Agent Horse".
With alternate spellings "Ostrhagin" becomes "Rani Ghost", "Osterhaigen Key" becomes "Rose the key again"
In German "Ost er hagen" translates to "East it cultivated".
The key could be the trigger for a Delta wave as seen in (DW: The Parting of the Ways); or alternatively have the Skasas Paradigm seen in School Reunion saved onto it and be the back plot for a reset of some elements of the series due to Russel T Davies' departure as an executive producer.