Just to make it clearer
1851 Address: Fore Street, Kingsbridge
OLDFIELD, William Head Married M 48 1803 Rope Maker Kingsbridge
OLDFIELD, Mary Wife Married F 44 1807 Shoe Binder Foway Cornwall
OLDFIELD, Mary J Daughter Unmarried F 18 1833 Kingsbridge Devonshire
OLDFIELD, Elizabeth Daughter Unmarried F 16 1835 Boot Binder Kingsbridge Devonshire
OLDFIELD, William G Son M 9 1842 Scholar Devonport
OLDFIELD, James Son M 1 1850 Kingsbridge
As Phillip was born before the marriage he has the name Oldfield.
In 1861 George and Mary are in Stoke Dameral and Phillip is visiting his grandparents. He has now taken the name of his mother's husband.
1861 Address: 6, Fellowes Place, Stoke Damerel, Devonport
OLDFIELD, William Head Married M 57 1804 Rope Maker Kingsbridge Devon
OLDFIELD, Mary Wife Married F 53 1808 Folvey Cornwall
OLDFIELD, John Son Unmarried M 11 1850 Kingsbridge Devon
DAVIS, Philip Visitor Unmarried M 9 1852 Kingsbridge Devon
In 1871 Mary Jane Davis is back home with her parents and Phillip has reverted to Oldfield.
1871 Address: Cornwall Street, Stoke Damerel, Devonport
OLDFIELD, William Head M 70 1801 Devon
OLDFIELD, Mary Wife F 62 1809 Cornwall
DAVIS, Mary J Daughter F 37 1834 Devon
OLDFIELD, Philip Grandson M 19 1852 Devon
after this time they both revert to Oldfield. You have found the death of a Mary Jane Davis but it does not say she was the wife of George Vaughn Davis.
LK