I had the same trouble with a new saw since I had oiled the arbor before
installing the blade... a no no.

Someone on the wreck advised a little liquid wrench and tapping the
nut(the metal nut, not the human nut that oiled the arbor) after
applying the liquid wrench. And find a longer handled wrench to fit the
nut if possible, while holding the other side of the arbor with another
wrench. I tried all and bingo... loose nut.

RonT