1. Freewill theism has a generic theodicy: the freewill defense. (That can be supplemented by other theodicies, like the soul-making theodicy.)
According to the argument from evil, an omnipotent, omniscient, and benevolent God is blameworthy for failing to preempt any preventable evil–especially gratuitous evil.
The freewill defense denies this key premise. God is not blameworthy for preempting every preventable evil, because the price of eliminating evil is to eliminate goods that are inseparable from libertarian freedom.
Open theism takes a different tack: God is not to blame because God lacks advance knowledge of evil actions. God didn't see it coming down the pike. That's what is distinctive to an open theist theodicy.
That, however, means that open theism implicitly concedes a key premise of the argument from evil, and thereby rejects the freewill defense: if evil were foreseeable, then God would be blameworthy for failing to prevent it.
2. However, that makes the open theist theodicy implausible. To begin with, the open theist God is like a man in a security room. The world is blanketed by surveillance cameras. Inside the security room are wall-to-wall screens which display what everyone is doing everywhere, at every moment.
So, for instance, God can see Ted Bundy incapacitate a hooker or coed, dump her in the trunk, and drive her to his hideout. Now even if God doesn't know for sure how that will end, isn't it enough to for him to see Bundy put a woman in the trunk? How much more do you need to see to intervene?
If a human observer saw that, and he was in a position to intervene, would he not be culpable for failing to rescue the woman? (And keep in mind that freewill theists use human analogies in objection to Calvinism.)
3. But it gets worse. Some open theists are more philosophically inclined while others are more exegetically inclined. The latter pride themselves on their fidelity to Scripture. They consider their interpretations to be more faithful, more straightforward, than classical theism.
Yet Scripture frequently says God is a mindreader. That tends to crop up in reference to God's qualifications as the eschatological judge.
God doesn't simply know what people do, but what they intend to do. But that means God's knowledge of human affairs isn't confined to what he can observe. In addition, God is right inside the mind of the serial killer or the suicide bomber.
Now, according to open theism, God can't know what we are thinking before we think it. God doesn't know what our next thought will be.
But he does know what people are planning to do. He doesn't have to wait and see what Bundy is going to do to that women. He has direct access to Bundy's mind.
Yet that makes it much harder for open theists to claim that moral evils are unforeseeable. Although there may be a bit of lag time in the sense that God doesn't know what evildoers are plotting ahead of time, his knowledge of their intentions is simultaneous with their intentions. He knows as much as the agent himself. Real time, up to the moment, intel. God is eavesdropping on their thoughts. He knows what they intend as soon as they intend it.
Surely that puts God in a position to head off ever so many moral evils in the making. He needn't wait until the last moment.