He was the youngest of the nine children of Ivor Bertie Guest, 1st Baron Wimborne (1835–1914) and his wife Lady Cornelia Henrietta Maria Spencer-Churchill (1847–1927), daughter of John Spencer-Churchill, 7th Duke of Marlborough, aunt of Sir Winston Churchill.
The Guest family were wealthy industrialists whose interests included the fastenings-manufacturing company Guest, Keen and Nettlefolds (now known as GKN). Oscar was elected at the 1918 general election as Liberal Party Member of Parliament for Loughborough in Leicestershire, but stood down at the 1922 general election. He did not stand for Parliament again until the 1935 general election, when he was elected as Conservative Member of Parliament for the Camberwell North West constituency in South London.
Oscar was defeated there, ironically by a much wider margin than the Labour majority in Camberwell North West.
31st United Kingdom Parliament. 37th United Kingdom Parliament]
He was twice elected as a Member of Parliament (Member of Parliament).
