[2] These grievances probably played a part in Guernsey's decision to declare for Parliament on the outbreak of the civil war, but Osborne remained loyal to the King, holding the impregnable Castle Cornet for the royal cause against constant siege.
The castle was strategically priceless, commanding the entrance to St Peter Port harbour and reinforcing the still Royalist Jersey.
He remained at Cornet Castle until 1646 when with the military phase of the war in England over, he appointed Sir Baldwin Wake as lieutenant-governor and handed over the fortress to him.
[3][4] Osborne seems first to have retired abroad, living in St Malo while his property was sequestered by Parliament, but later he returned, poverty-stricken, to Chicksands, where he died in 1653.
One of his daughters, Dorothy, married Sir William Temple, and is well known for her charming "Letters", which were edited by Judge Parry in 1888.