Oakeshott served in the Royal Navy from 1940 to 1945 on destroyer escort during and was relieved from service after being wounded. He returned to A.E. Johnson, Ltd. and served as its director for fifteen years before leaving in 1960 to become a full-time researcher and writer. In 1964 he was elected a Fellow of the Society of Antiquaries. He co-founded the Arms and Armour Society in 1948, for which he served as President in 1951. That same year, Oakeshott published the article "A Royal Sword in Westminster Abbey" in "The Connoisseur" on the results of his work on the sword of Henry V in Westminster Abbey. As a result, Oakeshott began to be consulted by museums such as the Fitzwilliam Museum at Cambridge and private collectors.