- Ginny is more present in this book than in the previous four combined, and often makes Cho look bad. It's Ginny's idea of "Dumbledore's Army" that is used, instead of Cho's more generic "Defense Association." Ginny is usually the one who snaps Harry out of his numerous funks over the course of the book, including when he's convinced he's being possessed by Voldemort. She has been possessed by him, and could have told Harry exactly what it felt like, if he had bothered to ask. And the tender scene in the library as Harry and Ginny share Easter eggs together. And the line "Ginny got the Snitch right out from under [Cho's] nose." Could it be more obvious? Especially the scene where the two share an Easter Egg?
- Neville also begins to show signs of life as well; notably, he's still fighting when all the other kids, who are notably better wizards than him, have already been defeated. Unfortunately, this just shows the danger in relying on Fridge Brilliance: Rowling did it all so subtly with both Neville and Ginny that a lot of people missed it.
- "I heard... that awful boy... telling her about them, years ago." Harry assumes she's taking about his father and snaps at her. That's not Aunt Petunia referring to James and Lily. She's referring to Severus Snape, who is revealed in the final book to have been living in Petunia and Lily's neighbourhood and had been friends with Lily long before she met James.
- Snape, when talking to Harry about Legilimency: "The Dark Lord, for instance, almost always knows when somebody is lying to him." He says "almost" because Snape is the one who (for a while at least) manages to get away with lying to Voldemort in the final two installments.
- Looking back at this quote, it can also be seen as foreshadowing to Naricssa successfully straight-up lying to Voldemort about Harry being dead in Deathly Hallows.
- There's a reason Sirius's name is blacked out.
- The full prophecy's final line: Neither can live while the other survives is misinterpreted by both Harry and deliberately falsified by Dumbledore, as meaning one must kill the other. It's actually a subtle foreshadowing of the fact that contained within Harry is a piece of Voldemort's soul, and he remains tied to Voldemort as a Horcrux.
- Early on Nearly-Headless Nick claims to have "never been guilty of cowardice in [his] life!" At the end he reveals that the reason he's a ghost is because he was afraid of death.
- Whilst Sirius and Harry are examining the Black family tree, they come across Phineas Nigellus Black. Sirius states that "[he's the] least popular Headmaster Hogwarts ever had". Later in this book, that title will go to Dolores Jane Umbridge.
- At Grimmauld Place, amid a Doxy infestation, Molly is noted to have consulted "Gilderoy Lockhart's Guide to Household Pests". Lockhart later makes his first appearance in the series since Book 2 when the trio visit St. Mungo's and it is revealed he is writing his final book "Who Am I?".
- At Grimmauld Place, the locket found in the drawing room has great significance in Book 7.
- The memory of Snape's that Harry watches has the younger Snape use some spell that leads to a slash appearing on James' cheek, like he was cut with a knife. That's Sectumsempra, a spell that doesn't get named until the next book, and is revealed to have been somewhat of a signature spell of Snape's. Also, James counterattack with a spell that is more than likely to be Levicorpus.
- At one point, Phineas Nigellus Black tells Harry that the Slytherins can be brave, but don't go headlong into danger like the Gryffindors—they were more likely to retreat and come back with help. This foretells the climax of not only this book (Harry rushes headlong into danger, a mistake that results in the death of his godfather), but also the climax of Book 7 (Professor Slughorn leaves with the evacuating students, including the rest of the Slytherins, but returns later with reinforcements in the form of the students' parents and the citizens of Hogsmeade, later going up against Voldemort himself. Word of God is that some of the Slytherin students also returned with Slughorn, also with reinforcements).
- Harry, concerned that Hagrid's injuries aren't going away, wonders whether he was attacked by something with magical venom that prevents wounds from healing. Later in that same chapter, Mr. Weasley is attacked by Nagini, and we find out that Nagini possesses that very ability.
- Sirius's younger brother is named Regulus, following the Stellar Theme Naming used by the Black Family. More precisely, he's named after the most prominent star in the Leo Constellation, also called Cor Leonis, the lion's heart. Now, what do we learn about why he died in Book 7?
- Ron and Hermione's bickering strongly reminds Harry of Molly and Arthur Weasley's bickering at one point. Molly and Arthur are an old married couple. Come the epilogue of Book 7...
- In the Hog's Head inn Harry notices that the bartender looks "vaguely familiar". It's Dumbledore's brother Aberforth.
- The chapter named "Snape's Worst Memory" is named not for the bullying Snape endures, but for another reason, which we learn in Book 7.
- The first American Editions cover art (on the backside) features the party of Order members who go to rescue Harrys group - Moody, Lupin, Tonks, and Sirius and provides two examples of this trope at once.
- Sirius is in an archway and isnt with the main three on the cover, foreshadowing how hell be passing through the veil.
- Kingsley being absent from the group also coincidentally foreshadows how hell be the only character from the five who survives the series.
Foreshadowing / Harry Potter and the Order of the Phoenix