bastion just punishes people who stand out in the open, just get an angle on him or a corner and you're good.
reaper can ghost behind him
mcree can stun him
having a widowmaker can really help see where he is and you can move accordingly
roadhog can hook him out of the turret
reinhardt can rocket pin him out
genji can reflect the bullets
junkrat can just lob bombs at him from cover or corners
winston can leap in, sheild then attack him
zarya can bubble sheild
hanzo has scatter arrow
tracer can run around easily
plus I'm sure the other characters have things they can do
the only "OP" thing about bastion is when his team is supporting him so you can't get to him