In Harry Potter and the Order of the Phoenix we're introduced to a new evil character: Dolores Umbridge. Professor Umbridge isn't in love with a particular belief system or an ideology. We mean, yes, she does believe that humans should be at the top of the magical hierarchy. But besides that, she doesn't seem that committed to a single idea. What she is in love with is power. There is nothing she enjoys more than demonstrating her power over other people. The thing that's interesting about Professor Umbridge's love of power is that she doesn't even have to work for Voldemort. By struggling for authority, she becomes so selfish that she empowers people like Draco, Crabbe, and Goyle, who are more closely connected to Voldemort. Professor Umbridge may not actively support Voldemort, but does that matter? Her persecution of Harry and Professor Dumbledore because they are threats to her power does Voldemort's work for him.
As Harry shouts to Professor Dumbledore after Sirius has died, "People don't like being locked up!" (37.131). In Harry Potter and the Order of the Phoenix, both Harry and Sirius spend long periods of time trapped where they don't want to be: Harry gets stuck with the Dursleys over the summer holidays (blerg) and Sirius must stay at Number Twelve, Grimmauld Place all year to avoid Voldemort and the Ministry. Both of them start to go stir crazy from the isolation. Harry, who is only stuck alone for the summer, contents himself with throwing tantrums. Sirius is the one who keeps going out in public even though he is a wanted murderer.
The theme of isolation in Book 5 is not limited to physical isolation. Harry also experiences intense social isolation because few people believe that he is telling the truth about Voldemort rising again. There's almost nothing as isolating as being thought crazy by pretty much everyone around you. What is more, Harry has a unique destiny. According to the prophecy, he is the one who has to kill Voldemort or be killed. That's a totally isolating experience: to be marked for a destiny that no one else around you can share.
Harry Potter lost both his parents when he was a year old. He was raised by emotionally and sometimes physically abusive people who essentially used him as a servant until he was eleven. When he joined the wizarding world, Harry found out that a crazed maniac has an unexplained, personal grudge against him. And Harry has faced that crazed maniac and his followers four times in four years. What's more, on the most recent occasion, at the end of Book 4, Harry had to watch one of his fellow students – a boy he liked and respected – get murdered. So yeah, there has been plenty of suffering in Harry's life up until Book 5. But it's in Order of the Phoenix that all of this suffering seems to be hitting Harry: the Dursleys, Cedric, Voldemort, being mistrusted, being singled out by the Ministry of Magic. His suffering takes on a huge thematic importance in this novel.
In the words of the great American novelist William Faulkner, "The past is not dead. It isn't even past." In other words, the past lives on in the present. It shapes every minute of our lives. And this continuation is utterly apparent in Harry Potter and the Order of the Phoenix. Professor Snape and Sirius are so unable to let go of their school rivalries that they can scarcely stand to look at each other, let alone work together. Sirius hates to be reminded of his childhood so much that he takes out his frustration on the house elf Kreacher, with disastrous results. On the more positive side, Harry's parents died many years before, but the protection of Harry's mother's blood endures to this day. The sacrifice of her life was so powerfully driven by love that it is still affecting Harry fourteen years later. Lily Potter may have died, but in some sense she is still part of Harry's present existence. If the past has a constant effect on the everyday lived experiences of the characters, how can we even call it "past"?
We've seen a lot of bigotry in the Harry Potter series before. Draco Malfoy calls Hermione a Mudblood, meaning that she's a magic-user with non-magical parents. Then there's the Death Eater "prank" at the beginning of Book 4, when they levitate a Muggle family through the campground at the Quidditch World Cup because they enjoy torturing Muggles. But the hatred in Harry Potter and the Order of the Phoenix has a slightly different character.
Professor Umbridge hates half-humans, so she also has racial prejudice. But her hatred is largely aimed at people who disrespect her authority. Professor Umbridge loves rules. When she first comes in to the classroom, she insists that the students answer her with a full, "Yes, Professor Umbridge" or "No, Professor Umbridge." She wants them to show their obedience to her at all times. And when a student – ahem, Harry – doesn't immediately obey her, she turns the most terrifying loathing onto them. The thing is, Professor Umbridge's name is notably close to "umbrage," meaning to take offense. Everything offends Professor Umbridge, and she hates anything that offends her. So, by association, Professor Umbridge pretty much hates everything. And her hatred sours the whole atmosphere of Hogwarts throughout Book 5.
If you've ever heard the expression "fair weather friend," you know that it's often easy to be friends with someone when things are going well (when the weather is fair, so to speak). Real friends will also stand by you when storms blow up in your life. Even at times when being your friend isn't much fun, the people who really love you will stick around anyway; that's what friendship is all about.
This may seem like a trite observation, but it does appear to be the theme of Harry's friendships in Order of the Phoenix. Harry is a giant walking ball of resentment in this book, but his best friends all endure his bad moods. Hermione and Ron both scold him for his temper tantrums, and even Ginny gets quite sharp with him when he's afraid he's being possessed by Voldemort. But they all put up with his foul temper, his defensiveness, and his sudden aggression because they love him and they trust that he'll snap out of it at some point. Luckily for him, he does – but not until after Sirius has died, which appears both tragic and terribly unfair.
Sirius has been thwarted for most of his life. He was accused of a murder he didn't commit and thrown into Azkaban for twelve years. When he finally broke out, he managed to track down the real murderer only to have him slip through Sirius's fingers to rejoin Voldemort. Now, Voldemort has discovered the secret of Sirius's ability to transform into a dog, so Sirius can't even go outside in his "Snuffles" shape. Instead, he is trapped inside his old family home, which is entirely filled with bitter memories of an ugly childhood. And rather than making the best of the situation – he's no longer in prison, he and Harry are forming a strong bond, etc. – Sirius hates where he is and what he is doing, and it clouds his judgment.
In Harry Potter and the Order of the Phoenix, Harry suffers similar feelings, though to a much smaller degree. So in some ways, Sirius's death could be a cautionary tale for Harry to be careful not to get too frustrated and dissatisfied – which sounds like a high price to pay for advice Harry could've gotten from 90% of self-help books out there.
Obviously, Harry Potter and the Order of the Phoenix is a novel about a school. So 99.9% of the characters are very young. The thematic significance of youth in Book 5 isn't limited to the simple fact that the characters are kids. What's more important is that a lot of Harry's troubles spring from his age. He is old enough to want to take control of his own life, but he is too young to be considered an adult in the wizarding world (where the age of maturity is seventeen). Harry is frustrated at the contradiction that he's faced-off with Voldemort numerous times, but he still gets stuck with the stupid Dursleys over his summer break.
Harry's age also means that a lot of people don't take him seriously: Mrs. Weasley tries to exclude him from all Order business. And as for Professor Umbridge, she would probably treat him like an actual baby if she had the chance. She is utterly condescending and disrespectful. This kind of treatment, whether well-meaning (Mrs. Weasley) or evil (Professor Umbridge) infuriates Harry because it reminds him that he is not yet in control of his own life. That's what stinks about late adolescence: you feel ready to take on the world, but the world doesn't think you're ready for it.