there are 2 possibilities of how they met: in high school, during the Spring Tournament, or as adults, maybe through mutual friends or by chance

the high school version: one fun idea is that Kenma is sitting somewhere wrapped in a hoody and playing on his phone and Osamu is not checking well enough, thinks it's Suna and sits down and begins talking to him, Kenma filters him out first, but that dude didn't stop talking so he finally looks up and Osamu finally realizes his mistake, he's so embarrassed, but he's also confident and shameless enough to apologize, introduce himself and then continue his "conversation" not like he was the only one talking the whole time

another idea is that he accompanies Atsumu to Karasuno, because his twin wants to talk to Kageyama and Hinata and Nekoma happens to be there as well, so when Atsumu is talking causing ruckus with KageHina, Osamu strikes up a conversation

so in any way, I see Osamu just keep talking to Kenma, who is unsure if he should just leave or not, but Osamu is not intense like Kageyama when he wanted to ask Kenma questions, but kind of fun, so he stays where he is and listens

Osamu would have to be the more straightforward person here and actively pursue Kenma (both in friendship and love)

so he's also the one confessing, he might actually do it rather quickly and casually he's screaming inside though when Inarizaki has to leave he goes up to Kenma, tells him he's interested in him and would like to get to know him better and if he's willing to exchange numbers to stay in contact

everybody -including Osamu- is surprised when Kenma is actually willing to exchange numbers, he just thinks it's easier to get to know each other virtually a little better first, before having to face Osamu in real life again

because they don't live too close to each other they have to wait a while before seeing each other again and until then they already developed a bond and caught feelings; Kenma swears it's a friendship they developed, so that his friends and teammates stop giggling and bothering him, while Osamu tells the whole Inarizaki team how they're basically already dating, they only need to talk it out and make it official

so when they do meet again Osamu is a little bit hurt, because Kenma is so awkward with him, but Kenma is actually overthinking and unsure what he wants and if he really thinks feels like this or if his friends messed with his head heart

so Osamu shifts down a gear, turns down the flirting and tries to just enjoy his time with Kenma even if it's just friendly

that's when Kenma notices this is not at all what he wants and he actually did enjoy the flirting and there's no way he wants to share Osamu and why is Lev talking to him?! He has to go and interfere!

So Kenma suddenly grabs Osamu, takes him somewhere more private as if both teams and everybody mildly involved aren't eavesdropping and hiding in bushes to not miss anything

so in the end it's Kenma who confesses, in the most annoyed tone possible, that he likes Osamu and the attention he gets from him and that he does not want Osamu to flirt with anybody else, because that's even more annoying big tsundere energy

Osamu is over the moon and can hardly keep his cool act up and not giggle and hide his blushing cheeks behind his hands

he might actually ask Kenma then and there if he can kiss him, Kenma might actually say yes, even though he's nervous and shy, just to have something over the others he sees as his rivals

the as adults version: they'd probably meet through mutual friends at a get together, or a party of the MSBY players, or at a game, where Osamu is either in private to watch or sells his Onigiri

Osamu would know who Kenma is, Kenma might know who Osamu is, but probably wouldn't care too much

again I think that Osamu would and would have to be the one who starts a conversation and actively get to know Kenma

and it would work, because Osamu has a quieter demeanor than his brother so Kenma wouldn't categorize him as annoyed upon first look

Osamu would make sure that he gets invited to meetings/gatherings where Kenma will be present and become aquaintance with him, before asking him on a proper date

when Kenma warily agrees, Osamu ask if he's okay with a date at his flat, where he would cook something for them or if he'd prefer going out somewhere

Kenma is more than fine with staying inside and be cooked for

the date went well, the food was amazing, they had great conversations and Kenma might have leaned against Osamu when they were sitting on the sofa watching a movie, which Kenma didn't catch anything from, because he was distracted by Osamu, his broad shoulders and wide chest, the warmth he radiated, his lips he unconsiously licked from time to time...

Osamu would notice Kenma staring, but not comment on it

I also think he would take confessing slower in this scenario, even though he knows that Kenma likes him too or is at least interested

after a few more dates, he finally would present Kenma with an apple pie and while he watches him eating it gleefully he would confess that he really likes Kenma and that he would be happy if he'd want to be his boyfriend and take all this a step further

Kenma wouldn't be shocked or surprised, but a little shy, but he would agree and confess that he also started to like Osamu

after the confession the first kiss would come rather quickly I think... the night of that day, when they say goodbye Osamu would ask if Kenma was okay with him kissing him goodbye and Kenma was. He was trying to hide his excitement and how eager he was to kiss Osamu, but would fail and Osamu would nearly implode because he thinks that seeing Kenma like that is the cutest thing possible it also strokes his ego

on the outside they seem like the chillest couple ever, Kenma is gaming and Osamu is snacking while he watches his boyfriend play

it's not always like that though, they both seem competetive, so when they play a game together Kenma is usually better at it, but that doesn't mean Osamu is not driven to win and that's when things can get pretty chaotic, they try to cheat, sabotage the other and when they win they rub it in and when they lose it wasn't fair

it's all fun and games, though, I don't think they'd really fight over this, even though a few argument might have come from that, but nothing too serious

Osamu is a cook and he seems to love it, so he'd love to cook for Kenma, make sure he does not forget to eat and eats enough, he also bakes him apple pie from time to time, but not too often to not spoil Kenma too much and so that it doesn't lose the flair of something special

speaking of spoiling: we all know time-skip-sugar-daddy-Kenma-who-sponsors-Hinata, I also think he would like to buy Osamu gifts, things that are practical, like kitchen equipment, clothes, stuff like that so nobody can tell him he spoils his boyfriend, because those were things he absolutely needed and Kenma just provided

out with others they are more on the quieter side, except when Atsumu is with them, because the twins together tend to be loud (and can sometimes be a little much for Kenma)