Basara's canon is a little strange in that it is not completely linear. I've written about this in more detail
here, but if that is too much to read just know that I'll be mixing together a bit of two different routes to fill in some gaping holes, and that my interpretation of the end of her route is a bit different from the usual fanon.
We aren't shown exactly what Oichi's life is like before the start of 2, but it is heavily implied that she was with her elder brother, Nobunaga Oda, until being married off to Nagamasa Azai. She is submissive and scared of her brother, and he treats her cruelly throughout the game, so it's implied that through his abuse she became the sad woman she is.
In the video game Sengoku BASARA 2, Oichi is introduced as the wife of Nagamasa Azai and the younger sister of Nobunaga Oda.
The first glimpse of Oichi we see is that of her wedding day. She has been married to Nagamasa Azai under the orders of her brother. It is a lovely day and Nagamasa is glowing while Oichi seems to be a bit more apprehensive. After the palanquin carrying her arrives, suddenly an army appears and attacks. It is the army of Yoshimoto Imagawa, who attacks to disrupt the powerful alliance between the Azai and Oda clans. And because he's butthurt he didn't get an invitation to the ceremony. Mostly the invitation thing though.
Nagamasa is infuriated by this, claiming it an evil to invade on such a good day, and quickly vanquishes Yoshimoto; Oichi praises him for this, and Nagamasa bursts with pride. They plan to restart the ceremony from the start. However! Before the ceremony, they receive a message from Xavi. It's an invitation to the Xavi chapel, where an eternal love ceremony would be held. But this is just a trap; Xavi intends to lure in more followers of his religion.
Flabbergasted by this, Nagamasa proclaims this an unforgivable evil, and they go to attack. Meanwhile, Mitsuhide, a vassal of Oichi's brother Nobunaga, speaks with her; it's revealed that the true intent behind Oichi's marriage is to carve out the Azai forces to attack the Asakura clan with fewer hindrances. Oichi is meant to assassinate her husband, but it's obvious that she's started to fall in love with him by her hesitance. She says that it is impossible to stand up against her brother, and hangs her head. Mitsuhide leaves.
During this, it's discovered that the marriage certificate has been replaced with a conversion certificate by Xavi. Nagamasa is angry and the two fight; at the end of the fight, Xavi vanquished, Oichi is saddened and claims that all around her are unhappy, and that's ok if Nagamasa hates Oichi for this. Nagamasa tells her that she shouldn't worry about things like that and gives her a flower. Oichi starts to cry, heart warmed by his kindness.
The ceremony is finally completed, and the two are wed. Oichi finds a bit of happiness in this, being with the man she quickly comes to love.
However, Nobunaga has other plans. Deeming Oichi's assassination attempt a failure by Mitsuhide's reports, the Oda clan moves to attack the Asakura. Nagamasa is horrified, and quickly annuls the pact formed via his and Oichi's marriage. Under the pretence of punishing the impudence of the Azai, Nobunaga manoeuvres his troops to Anegawa, the Azai's home.
It turns into a slaughterfield. The Oda troops quickly overwhelm them and Oichi is perfectly powerless, unable to do anything as she watches her husband's men die.
Oichi pleads with Nagamasa to apologize to her brother, but Nagamasa is stubborn; he claims that Nobunaga betrayed them by going on to attack Asakura, and that Nobunaga has turned away from honour. Mitsuhide enters the scene. He and Oichi fight, and he is killed after a short exchange. Nagamasa tells Oichi to withdraw, but she doesn't want to-- she wants to talk with her brother and to get him to stop attacking. She doesn't want her brother to kill her husband or herself. Before he can convince her to run away, Nobunaga appears.
Starting to plead with her brother does no good. He interrupts her straight away and tells her and Nagamasa that they do not offer second chances, and he aims his gun at Oichi, starting their brawl. Nagamasa and Oichi fight against Nobunaga, Oichi pleading with him all the while, and eventually Nagamasa cuts in, directing Nobunaga's attacks towards him. They talk and Nobunaga continues to be unforgiving. Eventually, one of Nobunaga's attacks hit Oichi and she is knocked over-- he raises his gun to fire at her, but before he can pull the trigger Nagamasa is in the way.
He is killed by the bullet intended for his wife, and Oichi watches him bleed out, completely horrified. Nobunaga says that this isn't bad either, referring to Oichi's distress, and tells her that this is her fault. She is the one who killed Nagamasa. Oichi takes this to heart, believing it to truly be her fault.
Oichi is whisked away by her brother and his troops; she is not allowed time to heal the wound left by the loss of her loved one and is instead thrown onto the battlefield, working for Nobunaga's conquest. Her miserable figure looms over the Oda clan's forces, and she can do little but weep and blame herself for her husband's death. It leaves her an empty shell; so empty that when her brother shoots by her head with his shotgun, she does not flinch.
During the attack on the Takeda, Oichi struggles to live, fighting her hardest. She believes in nothing but the idea that her husband must be hating her in the afterlife, that she has failed as a wife, that had she worked harder, Nagamasa may not have died. She kills her brother's enemies. At the end of the battle, she falls to the ground, weeping, apologizing to her dead husband.
Completely stuck is what Oichi is-- she is broken and beaten up, and thus unable to break away from her cruel brother's commands. She fights on, letting her brother use her as he sees fit, despite it ruining her bit by gradual bit-- because, you see, she believes she doesn't deserve any better. She wants to live, despite the hardships thrown her way; when her brother tells her to do as he says or die, she choses to do as he says.
Wailing that she can't do it when faced by Kenshin Uesugi, her brother urges her on; she says that it hurts, Nagamasa will never forgive her. This is her breaking point; this is the moment where it loops around and she starts to feel okay again.
Oichi starts to hear voices. The voice of her husband, in particular; she hears him praise her, tell her she's done well, and Oichi perks up. She likes to be praised by him, and thinks that fighting will make him happy, so she goes on to throw all of herself into the battle. She goes at it with a vigour none had ever seen her before. She fights in his name, giddy and excited to the point where she suggests they attack some peasants to suppress their uprisings. She's praised by many for her sudden change in attitude, but this does not affect her.
Going with her brother's wife, Nouhime, and his trusted vassal, Ranmaru, they approach the peasants and gradually cut them down. It's slow going, and Oichi begins to panic during it. Nagamasa's voice fades for her, and she's suddenly very keenly afraid that he will leave her. Nouhime and Ranmaru realize her panicked state and go to assist her, but it does nothing. Oichi continues to panic, egged on by the peasant girl Itsuki, and she faintly hears Nagamasa saying something. She strains to hear it, and when she does, she kills Itsuki. Their leader defeated, the peasants back down.
Ranmaru as well as Nouhime approach Oichi, alarmed by her behaviour. But she turns to them as though a doll on strings, claiming that Nagamasa has told her to kill everyone. And so, she does. Nouhime and Ranmaru die by her hand easily and quickly enough. Her sights become set on the next target: her brother. She wanders to the castle at Honnoji, unsure of why she wishes to kill him; unsure if it's for her own sake, for Nagamasa's, or for her brother himself.
When she and her brother meet, she tells him that Nagamasa wants his head; Nobunaga asks her who really wants it, and she insists that it's Nagamasa, not herself. But as the castle of Honnoji goes up in flames around them, it is revealed that Oichi herself wishes for it. She blames Nobunaga for what she's become, for how the people around her always end up. He denies responsibility and she asks for proof of that. She asks for him to beg for his life, fully caught up in a moment of her true demonic nature. She kills him without mercy, watching him fall to the ground in a final heap.
Cheering, Oichi raises her hands to the heavens, gleefully dancing around. At last! Oichi has avenged her husband and her own ruined life! But this happiness does not last. She crumples to the ground in tears, realizing the emptiness of such a thing; Nagamasa hasn't been brought back to life; it changes nothing; it brings her no peace; and she feels as though a monster, having killed her own brother who she loved despite all the torment he put her through.
Escaping the burning castle, Oichi's mind breaks to an extreme it had not before. She gradually suppresses everything-- Nagamasa's kindness and death, her revenge on her brother, killing Nouhime and Ranmaru... everything. She wanders Japan as an amnesiac, unsure of her life's purpose and feeling a hole in her heart without knowing why.