Wikipedia The supposed tradition that the original oak was associated with a witch named Sarah or Sally is without foundation, and is likely to have arisen as a means of explaining what may have been a variant and local pronunciation of the name as 'Sally' Oak. Indeed, the name is actually recorded as Sally Oak on a canal map produced by John Snape in 1789. https://en.wikipedia.org/wiki/Selly_Oak#The_'Oak'_tree
,,, & Selly Oak is recorded in the Domesday Book as Escelie. The name Selly is derived from variants of "scelf-lei" or shelf-meadow, that is, pasture land on a shelf or terrace of land, probably the glacial deposits formed after the creation and later dispersal of Lake Harrison during the Quaternary period. Another source for the name comes from the Old English 'sele' meaning a building, or a hall. https://en.wikipedia.org/wiki/Selly_Oak#Toponymy