Arthur A'Bland was accused of a murder he did not commit and was forced to flee into Sherwood Forest where he sought sanctuary among the outlaws. Having left his children, Oswald and Alys behind, they were forced into serfdom by the local lord.
Oswald knew that even though he was just a boy, he was a free man. And so he rebelled against his servitude for which he was punished by being chained in the dungeon.
Alys came up with a plan of escape and they fled into the forest to find their father. Eventually Arthur's name was cleared and the family was able to return to their home.
When Oswald came of age, he married and had children of his own. And his family eventually dropped the surname of A'Bland and took up the name of their founding father - Oswald.
The lineage continued unbroken into the 20th Century where a young woman would be born into the Oswald family by the name of Clara. And in the new millennium she would be known as "The Impossible Girl" who was born to save the Time Lord known as the Doctor.
UPDATE: This may be the reason why Clara was so keen on meeting Robin Hood, to meet the legendary hero who helped save her ancestors. However, because she and the 12th Doctor met a Robin Hood (in "Robot Of Sherwood") who looked nothing like the official Robin Hood of Toobworld, it would appear that the TARDIS brought them into an alternate dimension. (The TARDIS doesn't always bring the Doctor where he wants to go, but always to where he needs to go.....)