I dont know about tops. But in pants Ian just grew out of his 6-12 months. I find their stuff is hard to predict some of it is too big and some runs smaller.
huge, imo. aidan (2.5) still wears 12-18 months shirts and shorts from Old Navy. (i only know bc sawyer got a ton for her bday and she's way too small and aidan fits in them perfectly)
Re: Old Navy Sizing?
I have also found that they run a little small. Personally, I always buy the bigger size.
ETA: You can always check their sizing charts online.