The problem with searching for the name is the number of variations there are for both the surname and first name.. as you've no doubt already seen the O' prefix can appear and reappear, as can the u and/or the g - i.e. O'Donoghue vs Donohoe etc
There quite a number of listings for the name Anastasia on the census returns but I suspect that a few may have reverted to a shortened version - i.e. Anna/Ann. Among the Anastasia listings there is quite a wide variation in spelling, or transcription - i.e. Annastisa, Anastiasia, Anistesia etc
Shane