 During the Welsh Wars of King Edward I Otto was very active diplomatically and militarily, beginning with the siege of Dolforwyn Castle in April 1277.On behalf of Edward, he concluded the Treaty of Aberconwy in November that brought the invasion of Wales in 1277 to an end.In 1278, he was appointed King's Lieutenant in the Duchy by Edward and sent to the Duchy of Gascony along with Robert Burnel.