One of my random pocket headcanons for the Dragnor gang in Conquest is that Ranmaru and Oichi are friends. They got to a point where they genuinely enjoy hanging out and just yapping away, and Oichi even managed to have normal conversations with Ranmaru (by normal conversations, I mean anything that isn't just him going on simping monologues about everything Nobunaga).
The ugly truth behind this friendship, however, is that - as much as they became real friends later on - Ranmaru originally only talked to Oichi because she's Nobunaga's sister and he figured that being on good terms with Nobunaga's closest people and loved ones a) was good court etiquette and b) would make Nobunaga like him more. If Oichi is/was ever aware of this, she has chosen to quietly forgive him. As we know, Ranmaru is both a really sweet and also very honest and earnest guy by nature, so even if he ever intended to be deceptive about it, he failed miserably. He can only do genuine.
This makes it quite likely that, before leaving the kingdom and journeying through Ransei, Oichi had a conversation with Ranmaru about Nobunaga's increasingly worrying behavior. Unfortunately for her, Ranmaru is the number one Nobunaga supporter and enabler, so even if he agreed on some things and worried about his lord as well, he made it clear that he was unwilling to do anything about it if that meant going against Nobunaga in any way. Oichi then decided to leave him behind as well.
All of this provides some additional context for the pre-Dragnor battle cutscene, naturally. But my headcanon about that is also that Ranmaru was aware of how hurt Nobunaga was when he discovered Oichi's "betrayal", and him slipping out of the gate and trying to talk her into coming back was a desperate last attempt to somehow smooth things over (before Nobunaga notices the enemy army with his sister in tow at his doorstep) and/or soften the blow. As we all know, Ranmaru failed due to Oichi refusing to give up literally everything she's fought for this entire time, and then Nobunaga crashed the party.
They go back to being friends after the events of Conquest. While with Hero/Heroine's army, Oichi also speaks warmly of Ranmaru, both during and after the main story. Once all is over, she scolds him for enabling and blindly following Nobunaga this whole time, but he is politely unrepentant.