As Gaen puts it together, they needed to take care of two problems. First they needed to establish someone as the new God. Recalling that in
Nadeko Medusa, Sengoku ended up being that God. It wasn't until she was saved by Kaiki that the position opened up again. The other issue was to eliminate Ougi Oshino.
Getting all the facts in order. When Shinobu came to this town, she ended up destroying both the God and the shrine. Which left a void that needed replacement for around 15 years. Logically speaking, it would make sense to appoint Shinobu as the new God. She fills the requirements needed as she had her first minion under her by the name of Seishirou Shishirui.
Now, normally this would be a no-brainer, but Koyomi performed a miracle and brought back Hachikuji from hell. Being a God can be seen as the ability to perform miracles. Hachikuji coming back is a miracle which keeps her from being swallowed by the Darkness or going back to hell. Now Hachikuji is all for this as her and Koyomi's relationship is very sweet. The question is well look at Sengoku, what happens if becoming a God changes her. The difference is that the apparition affecting Sengoku was a Snake. The snake consumed her as she became the Snake. However, for Hachikuji she is the snail apparition and the snail beats the snake. So she won't get consumed by the snake, so the issue of Sengoku won't happen here.
Next is the subject of Ougi Oshino. Gaen eludes that she is a monster, but is unknown. One thing that is a fact is that she is not Meme Oshino's niece. She isn't exactly the darkness, but a quasi-darkness. Much like how Koyomi is a quasi-vampire. In her mind, she brings balance much like the darkness does, but doesn't fulfil the role properly. The Darkness which we are aware of just wants everyone to fulfil their roles. It Ougi doesn't do exactly that that implies there is an edge case where even if everyone full fillies their roles and are happy, she will wrong fix what doesn't need to be fixed. Which is what makes her a danger according to Gaen.
The big shock is when Gaen tells Koyomi, and he reveals to us that the identity of Ougi Oshino is Koyomi Araragi. The problem is hold up how can this be? Well, Gaen makes the connection that Koyomi is similar to her older sister and Kanbaru's mom. Adding onto that all the things she has done in the past 6 months. She helped Koyomi resolve the incident of why Koyomi shut himself off. She forcefully resolved the dilemma with Sengoku where her obsession over him was unhealthy. Her becoming a God was the path for her to be saved. We had Koyomi facing Oikura his childhood friend and the mistake he made back then.
As we realized when Koyomi tackled the darkness which leads Hachikuji to reside her presence and move on to hell. This is where Koyomi learns of the phenomenon that corrects mistakes. Consider the fact that he couldn't save Hachikuji. It was him criticizing himself. In which the apparition known as Ougi Oshino would aid Koyomi in what he blames himself for. He blamed himself for what happened to Oikura and how he never helped her. He blamed himself for never really helping Sengoku get over her unhealthy obsession with him. The funny part is that Oshino last name did not come from Meme Oshino, but Shinobu. It's easy to forget that she is labeled Shinobu Oshino.
Then we have various cases with each character that Koyomi criticized and blamed himself.
- When he first meets Kiss-Shot Acerola-Orion Heart-Under-Blade he saves her out of sympathy. He realized that is not what she truly wanted as she didn't want to die, but was lonely. He wanted to make up to her as he became her partner.
- Hanekawa was Koyomi's first friend since the incident involving Oikura. She did so much to help him, but he continued to think if he deserves a friend like this.
- For Senjougahara given that he saved her, she grew fond and fell in love with him. Koyomi criticized himself since he felt he took advantage of the situation.
- With Kanbaru they had a fun dynamic, but he felt guilty he could never be like her. She was very straightforward and emotional manner.
- With Sengoku while he did help her, if you recall from the Nadeko Snake in Bakemonogatari, he was able to save her, but not the two other classmates.
- With Shinobu as they became closer, he criticized how he got these Vampire powers and whether if he deserved them or not.
All of these things, one by one, the apparition known as Ougi Oshino helped with. She was the one who dealt with the other adults which had Gaen to go after her. The constant criticism that Koyomi gave himself she was the one that fixed it and had him tackle it. The reason Gaen called her a quasi-darkness was that she full fillies the role, but only towards Koyomi's criticism of himself. As Gaen puts is he needs to tackle Ougi himself. He needs to tackle the criticism so he can grow up.
When push came to shove, Koyomi couldn't kill Ougi. Time after time, the people that Koyomi cared about saved him in so many ways. The criticisms he had regarding their relationships brought him back up. Now as Ougi represents Koyomi, will he simply not save himself? Doing that would betray the actions of those who he held dear to him.
Thanks to Hanekawa, who brought Meme Oshino back, he was able to save Ougi Oshino. Calling her his niece. Was this a bluff or the truth? Given that Meme Oshino has been away since
Bakemonogatari it is hard to say. Now Ougi Oshino has a place as Koyomi's underclassman, so the darkness couldn't remove her. Koyomi finally learns the lesson of he needs to love and take care of himself. He didn't care what happened to himself if so those that he cared about were ok. This was something he was unable to accept as he was willing to suffer eternally so that Hachikuji didn't have to leave. He was willing to go convince Sengoku when he was a God, and he used his powers if it just meant there was a chance to convince her, and he was never successful.
This was an important lesson for Koyomi. The big lessons for him to become an adult is to depend on others and learning to love himself. These are lessons that anyone can really relate to.
As Owarimonogatari wraps up, we see that all the adults are busy and gone again. As graduation approaches, this symbolizes Koyomi, Hanekawa and Hitagi slowly becoming adults. Koyomi and Hitagi becoming even closer as they are being teased by Hanekawa and Hanekawa doing what she truly wants to do.