then rod of ages
You get it that late? ROA is only really good if you get it before the 20min mark as the passive takes too long to take effect otherwise. I've used ROA on Morgana in the past, a long time ago but I really didn't see the ups to it when she doesn't need to be tanky at all and lets face it, ROA costs a ton.
If you are snowballing hard, just go straight Deathcap instead. Your AP will be insane.