You ever think about how Ouma actually did want to trust everyone so damn badly? I do.
It's in the passive back seat he took in Chapter One, where he allowed Akamatsu to take the wheel and only stepped in when things got really bad and they needed to pivot (see: The Death Road of Despair).
It's in the way he easily gave in to the peer pressure of everyone else wanting to use the Flashback Lights when he himself was against it.
It's in the fact that rather than agree with everyone that the Motive Videos shouldn't be watched, rather than try to steal them all to hoard and keep away from everyone or outright destroy them so no one could watch them, he suggested watching them all together as a group, so they could all hold each other accountable.
And even as that trust was slowly and methodically broken down by the killing game, even as he continues to lie and starts to act like he doesn't trust anyone and nobody should trust him either, he still wants to trust them so, so bad.
It's in the fact that even though he outed Harukawa for her lie, and made jabs at her expense, he passively allowed Momota to handle the situation without much of a fight and even walked back on his claim that he agreed with everyone who wanted to keep her away, and didn't question her presence in the group again after that, almost as if her reintegration into the group and everyone reestablishing trust with her was his goal from the start.
It's in the concern he had over the dangers of cult mentality and Yonaga's affect on the members of the student council.
It's in his open admission to how he was injured when he tripped on the loose floorboard during the investigation, rather than insisting the blood was fake and that the entire thing was a lie.
It's in the way he was the first to notice and properly call out Yumeno's aversion to confronting and processing her emotions over Yonaga and Chabashira's death, something even Chabashira herself failed to do when she was alive.
It's all over his entire relationship with Iruma, commissioning his tools from her and including her in his plans up until she falls to the pressure of the killing game herself.
It's in his relationship with Gokuhara up until the very end, the one remaining person who trusted him in return, and his absolute devastation over losing him.
It's in the loss of Gokuhara being the straw that broke the camel's back, dropping him entirely into a pit of despair and paranoia and resolution to keep everyone as far away from him as he possibly can.
And even then...
It's in the fact when he enacts the Mastermind plan, he takes Momota with him, arguably the one who's openly disagreed with him the most out of anyone.
It's in the way his final, last-ditch plan invaluably relied on his trust in others. Not just Momota, who had the plausible deniability of blackmail, but Saihara, too, to see through everything and figure out what Ouma was trying to do before it was too late.
He wanted to trust everyone. He wanted to trust and befriend everyone so, so, so badly.
But the killing game punished him every time he tried.
