"Wearing the 45" is also referring to the fact that if it is necessary for Jay-Z to return following a disagreement he will be carrying a "four-five" which is slang for a .45 caliber handgun.
H.O. is a shortened version of HOVA, Jay Z's nickname... Rap's Grateful Dead refers to the fact that he has an immense following. Much like the large following of the rock band Grateful Dead.
Obviously this is an ode to Michael Jordan's return to basketball after retirement, which was to be Jay's fate as well. But he's also talking about coming back, wearin the 45, meaning a gun
Michael Jordan took a break from basketball when his father died and he wore the #45 to honor is father.