The word has always had a t sound since Old English, and it's part of the reconstructed language Proto-Germanic in the form *ufta. Every other Germanic language displays a t in the corresponding word:
Scots oftin (“often”), North Frisian oftem (“often”), Saterland Frisian oafte (“often”), German oft (“often”), Pennsylvania German oft (“often”), Danish ofte (“often”), Norwegian Bokmål ofte (“often”), Norwegian Nynorsk ofte (“often”), Swedish ofta (“often”), and Icelandic oft (“often”).
It is, in fact, the opposite and in very simplified terms, just a book of how people currently pronounce words and their meaning today. Think of it more as a record book for the time it was printed, rather than a rule book; living languages are funny like that.
If you would like to know more, I highly recommend Word by Word written by Kory Stamper, one of the editors for the Merriam-Websters Dictionary.
You and J live in a bubble in a hyperloop tunnel. You have not invented “oft” as part of any language yet. Or do You and K live in the anthro and say oof… and laugh every time someone trips on a coconut often enough it becomes mean? Either is possible, yet oft misunderstood.