"So you're an oracle?" Piper asked. "You can tell the future?"
"More like the future mugs me from time to time," Rachel said. "I speak prophecies. The oracle's spirit kind of hijacks me every once in a while and speaks important stuff that doesn't make any sense to anybody. But yeah, the prophecies tell the future." (4.216-217)
In some sense, the future mugs everybody, whether they're oracles or not. And often the future doesn't make any sense, at least at first. So Rachel isn't that much different than anybody else—except maybe that she gets hijacked by other people's futures, rather than just by her own.
"Then after the war, things started to go wrong. Cabin Nine's chariots blew up. Their automatons went haywire. Their inventions started to malfunction. It was like a curse, and eventually people started calling it that—the Curse of Cabin Nine." (6.5)
The Curse of Cabin Nine is never really explained all that well. Is there really a curse, or is it just chance? And if there is a curse, what caused it? Is it because Leo hasn't shown up yet, and they need him to lead for things to be right? Fate here almost seems like a plot device—a way to make Leo even more important, and give him a chance to save all his siblings from bad karma.
"I cannot destroy you yet," the woman murmured. "The Fates will not allow it. But they do not protect your mother, and they cannot stop me from breaking your spirit. Remember this night, little hero, when they ask you to oppose me." (11.42)
The Fates who will not allow it might actually be the author in disguise. Riordan needs some way to explain why the bad guys—who seem to know everyone's destiny—didn't just rub out the heroes in their infancy, after all. Fate and prophecy present all sorts of problems. If everybody knows what's going to happen, then why not change it? And if it can't be changed, then why bother trying ever?
"It's destiny, cupcakes!" Hedge insisted. "I'm meant to protect you. What's the quest?" (31.78)
Hedge is deluding himself here—he's not really part of the prophecies. He's just a hanger-on. At the same time, though, everything in the book is meant to happen in some sense—it's all been planned out and plotted. There really is a force behind everything, somebody has made sure that the foreshadowing is right and that the prophecies come true. Hedge is as much a part of the plot as anyone else, so even though he isn't directly connected to the prophecies, he is following a predetermined path (set by the author).
"Why are we here?"
Leo sniffed. "That's what I asked him."
Jason gazed into the storm as if watching for something. "That glittery wind trail we saw yesterday? It was still in the sky, though it had faded a lot. I followed it until I couldn't see it anymore. Then—honestly I'm not sure. I just felt like this was the right place to stop." (33.29-31)
When the book isn't quite sure how to get the heroes from point A to point B, it often throws in mystical feelings like Jason gets here. It's as if the heroes get brief communications from the author himself—a mystical force more powerful than all those gods and giants.
Now Aeolus had returned the picture. Leo knew that meant his destiny was getting close; but the journey was as frustrating as this stupid mountain. Every time Leo thought they'd reached the summit, it turned out to be just another ridge with an even higher one behind it. (41.32)
The description of destiny here is also a description of the book, which is structured as one encounter after another, each one more difficult than the last.
"My fate is preordained," Enceladus said. "Giants cannot be killed by gods or heroes."
"Only by both," Jason said. The giant's smile faltered, and Jason saw in his eyes something like fear. (44.15-16)
This sounds less like fate than like ground rules. It's like saying, "my fate is preordained— I can only be struck out if I miss three pitches!" That is how fate works in The Lost Hero, though. Destiny almost seems like an instruction list—get the seven heroes in the boat by the solstice and paint them blue, and the kangaroo of truth will appear with a toothbrush of great power. Or something like that.
"And Jason—you are wiser than your sister. I chose my champion well."
"I'm not your champion, lady," Jason said. "I'm only helping you because you stole my memories and you're better than the alternative." (48.61-62)
Hera is saying that Jason's destiny is to be her champion, but Jason is quick to respond by saying he's only helping her because she blackmailed him. Do you think this is this fate or free will? We think it depends on how you look at it.
"You were given a destiny," Hera said. "You were given into my service."
Jason scowled. "Because you forced my mom to do that."(51.48-49)
Again, Hera says destiny while Jason says blackmail. They seem to agree, though, that Jason's mom made a decision that has impacted the rest of his life. Family is destiny—which, as Jason suggests, can be irritating.
"It's a diagram for a flying ship. […] Was it ever made?"
"Not yet," Leo said. (53.43-44)
You could see diagrams as a kind of destiny or fate: you lay out what's going to happen, and then it happens. A blueprint is a kind of oracle, though without the green smoke and the rhymes.