The young Otto travelled to England probably in the company of
Peter II of Savoy by 1252, certainly not later than 1265. His father, a loyal ally of Peter was receiving payments from the English crown from 1245. There he entered the service of King
Henry III and by 1267 was placed in the household of Prince Edward. In return for likely service for Prince Edward at the battles of Lewes and Evesham he was rewarded with property at
Queenhithe in London. In 1268 both prince and servant were knighted and in 1271 the latter accompanied his lord on the
Ninth Crusade, where he served at Acre that year. According to Jean d’Ypres, it was Otto, not
Eleanor of Castile, who sucked the poison from the wounded Edward after an attempted assassination. In 1272 Otto was appointed an executor in Acre. In 1277 he was appointed lord of the
Channel Islands and in 1290 appointed a
bailiff for Jersey and
another for Guernsey, giving them civil powers to administer the islands. However, given his many duties for Edward, Grandson would be an absentee landlord. Those appointed by him receiving some criticism. Grandson would make only one certain visit to the islands, this in 1323. Returning to England, he was a key
household knight of King Edward I in his campaigns in
Scotland and
Wales, where he served as
chief justiciar of Wales, based at
Caernarfon Castle from 1284 to 1294 - although his time in Caernarfon was limited. 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. In a letter attributed to him from
Dolforwyn Castle he may well have been instrumental in bringing Master
James of Saint George to Wales - the man who would later build
Flint Castle,
Rhuddlan Castle,
Conwy Castle,
Caernarfon Castle,
Harlech Castle and
Beaumaris Castle. Indeed Grandson is recorded as directing early palisade works at
Flint Castle. On behalf of Edward, he concluded the
Treaty of Aberconwy in November that brought the invasion of Wales in 1277 to an end. In early 1278, he was sent to
Gascony by Edward along with
Robert Burnel, to reform the government - Grandson and Burnell were joint Lieutenants of Gascony from 7 February 1278 until September 1278. They appointed
Jean I de Grailly as the new
Seneschal and laid the foundations for the
Treaty of Amiens sealed in 1279. He was also employed as a diplomat and gained contacts with most of the sovereigns of western Europe. During the second invasion of Wales in 1282–83 he narrowly escaped death at the
battle of Moel-y-don before in April 1283 taking the town of
Harlech at the head of 560 infantry. As a commander of the royal army that had begun the campaign from Anglesey he was amongst the first of Edward’s retinue to see the future castle sites at
Caernarfon and
Harlech. In 1283 he was briefly in the employ of
Edmund Crouchback, the king's younger brother, for diplomatic work. It was said that no one could do the king's will better, including the king himself. ==Crusading years (1290–95)==