Olive's family history is difficult to trace because her father was apparently decended from a French emigré noble family, the barons and viscounts of Thoren, which seems to have had branches all over the world. It's also difficult 'cos that surname's a b*gger to search for - spelt differently every time!
Her father Oscar William dSdT was apparently born in Worcester, but was a naval officer so probably travelled a great deal, and mother Alice Drury appears to have been born in Indonesia.
Perhaps they only lived at Cheltenham for a few years.
Oscar William appears on the 1891 census living alone at 10 Alhambra Road, Portsea, Portsmouth, age 47, "Retired Cap. Navy, Now Principal Of The Jersey & Southsea Military Educational Establishment". I can't find any trace of his wife and kids at that time. Overseas?
Millicent dSdT, who must be Oscar W's mother, died at Portsea in 1884 aged 77.
(N.B. Oscar William dSdT should not be confused with Oscar John dSdT, who must be a cousin. He was a sheep farmer in Australia for many years before retiring to Folkestone, Kent, in 1882, where he appears on the 1891 and 1901 censuses. There's a biography of him here: http://www.adb.online.anu.edu.au/biogs/A040058b.htm
Olive's 2-years-elder sister Violet Alice Ione was also born in Cheltenham in 1874, but her 2-years-younger brother Lionel Aymar was born at Portsea (1878).
Can't find any DoBs in England & Wales for her other siblings, Oscar Ellis, Nina Katherine and Wanda Beatrice, - overseas perhaps?
By 1914, Olive was 38 and unmarried. Oscar William had died in 1900, Alice probably not long after, Violet, Lionel and Nina were married, Oscar Ellis and Wanda - maybe married or overseas or both.
So I would guess that Olive is commemorated in Hertingfordbury because she was living there at the time - or at least before the war.
Maybe John has some more information, like where she died - I can't find her on CWCG.