Swift's spelling of the word was not what I had expected; but what do you think?
The correct spelling is "ockster"; Swift is correct and nwhyte's instincts were wrong..
The correct spelling is "oxter"; Swift's spelling is archaic and nwhyte's instincts were correct.
Both "ockster" and "oxter" are acceptable spellings.
Neither "ockster" nor "oxter" is an acceptable spelling, and I will give what I believe to be the correct spelling in comments.
I have never encountered the word before and thus have no view on how it should be spelt.
Edited to add: Ken MacLeod agrees with me rather than with Jonathan Swift.