Timber Hill (two words), previously Timberhill Street, existed and still exists.
http://www.georgeplunkett.co.uk/Norwich/tel.htm#Timbe
Osborne Square with Osborne's Building, was on the East side of Timber Hill.
Sources "The Old Courts and Yards of Norwich" by Frances and Michael Holmes and 1905 OS Map.
Oak Street is a completely different place, with similarly named Osborne's Yards leading off it.