[1] He was the son of John Wickins, vicar of Petworth, Sussex, and his wife Philadelphia Mitford.
[1] He inherited in 1770 estates from Fountayne Wentworth Osbaldeston, who was his maternal great-uncle, and changed his surname accordingly.
Shortly before that he had been admitted, age 16, to St John's College, Cambridge; he matriculated there in 1773, graduating Master of Arts (MA) per literas regias.
[1] Osbaldeston went into politics with the backing of the Fox–North Coalition in November 1783, ahead of the 1784 British general election.
He stood for the local set of Scarborough, rather than the rotten borough offered him, Hedon, and was elected.