The first thing we need to remember is the fact that no one knows where Ock's body is right now. When Carlie Cooper was investigating the possiblity of a brain-swap situation, she got to Otto's grave only to fall through the earth into the Green Goblins lair. With that in mind it wouldn't be too much of a stretch for him to return with the help of one of the Marvel Universe's many magic fans, from Doctor Strange to Magik. Upon his return, Ock's mind could be transferred back into his body using the golden octobot he used last time, however the last time he took a back-up was before The Amazing Spider-Man #698 so it would mean him returning with no recollection of his redemption and taking up his role as big bad once more. The problem with this theory is that he died of natural causes and to resurrect him wouldn't do much to help his aged, failing body which leads us to my next theory...