Oda is one of the bad guys, while Ango is supposedly on the right side.
In all truth, this division seems more dishonest and blurry with every day. Over the way too short course of their friendship, Ango has lived enough identities all over the moral compass to lose sight of his actual position on it. Was there anything that made his righteous self inherently true? How was he to decide? He had enough actions to prove any of hims real. He wondered if his friends also feel that dissonance - if they, too, consist of an array of splintered identities, contradicting each other and preventing any clear judgement from being made.
Ango tries to carry himself with elegance when Oda really, truly, doesn't care.
Taking care of his appearance is as natural to Ango as breathing. The grey suit is his second skin, his hair is neatly combed back and the expensive cologne he wears keeps the smell of dust and old papers at bay. Even his own genes seem to be on his side here, because there is an actual, goddamn beauty mark on his face. He likes tending to himself -it makes him feel better. Like he has control over something. Even if things are falling apart all around him, he still has those mornings when he can throw water over his face, look at himself in the mirror and say that things are okay - even when they are anything but.
Oda, more often than not, presented himself in a casual, disheveled way that suggests that he cares enough to buy nice clothes, but not nearly enough to keep them in condition to match (Ango once caught him with a yellow post-it note stuck to his jacket - a courtesy of his kids, as Oda himself quickly deduced). He smells of sweat and curry and his slightly ruffled hair and the ever-present stubble always makes him look like he walked out of his flat in a hurry.
However, if someone looked close enough, scratched beneath that layer of first impression, they would probably notice sickly paleness of Ango's skin, with features looking like they've been molded in wax and dark circles under the eyes - and those eyes themselves, like marbles of opaque glass behing drooping eyelids. A clever onlooker could also spot the odd shapes his otherwise elegant suit took when it hanged off his bony frame ("malnourished?", you could think and you wouldn't be wrong. Ango didn't have much regard for his own well being, often staying awake only thanks to the sheer force of coffee and paranoia). His movements were stiff, with no grace or composure - rapid, almost skittish. It drives people away - surely, they must sense a counterfeit. Ango tells himself it's better that way when he sinks into his archives - until one day he can no more, because those two idiots walk into his life.
At the same time, there is something about Oda that makes people trust him. It's hard to pinpoint what it is exactly - he's kind, sure, but not in a naive, cheery, ostentacious way. He doesn't even smile a lot. And yet that elusive something is undeniably there - after all, he was the one to tame and understand the fearsome Osamu Dazai and see humanity in the one who was No Longer Human. Perhaps it was his honesty, be it with friendliness or disdain. Yes, he could hold a knife to your throat, but he would never use it to stab you in the back. His flaws? Only serve to make him more approachable.
Oda seems like an imperfect human.
Ango seems like a perfect imitation of a human.
Oda is looking forward towards the future, while Ango is forever stuck in the past.
Ango lives his life burdened by what was, his lungs filling with dust and moths living in his hair. Ash covers all of him, turning him grey and dull and condemning him to fate of a ghost, not unlike those whose lifes he's been recording in the Mafia. Even his ability, the very one that makes him so good at what he does, relies on uncovering the past. He exists surrounded by flashes of memories and pieces of information, but not much grip on where it would lead him. He could wake up dead tomorrow for all he knows.
Oda has no use for the past. He has too much responsibilities to let himself dwell. He has both a reason and a way to plan his future - because where his ability will not take him, he will get himself with the power of will, determined to find his way to a house with a seaside view, five kids and a desk with an unfinished novel draft lying in the drawer.
Oda is strong when Ango is weak.
And he comes for him. Oda Sakunosuke comes for Ango Sakaguchi when he's at his weakest and most helpless, tied to a chair and about to burn down. Maybe it would've been for the better. Ango has always been a creature of dust and it would've been fair for him to turn to dust on that fateful night. And he was sure he will - a sacrifice he was ready to make all along. Instead he is found by Oda.
When the backup arrives and Ango is free to walk away into the night, he thinks about how easy it would be to arrest Oda on the spot - and about how hard it would be at the same time. Hard, because that man still had so much power over him, without even intending to, and Ango was not sure he will ever be quite able to shake it off. Serves him right for finding companionship where he shouldn't.
(He leaves Oda there, of course he does. That's the most he can do at that time. A few days later, at Lupin, he's already at peace with the thought that he will see him for the last time - though he's not aware how much more there is to this prophecy until it's too late.)
Oda can afford to be warm, smooth and soft where Ango forces himself to be sharp and shivers from the cold. Oda's firm when it's needed, while Ango crumbles too easily, useless like a sword made of glass.
And yet, in the end, when all the dust has settled, it's Ango who lives when Oda is dead.