I’m assuming at this point that at least some of you have seen essie’s theory on how and why Osamu could’ve been a secondary partner to Milleniummon? Ya know, this one? [x]. If you haven’t, go read it! It’s good! If you haven’t and also you don’t want to, well first off, mean, but tl;dr: Milleniummon is a jogress which implies only one of his halves originally belonged to Ryou, and because Ken wasn’t intentionally made a chosen, that means the digivice that appeared in the Ichijouji household could’ve been Osamu’s. Connect the dots and you get a baseless but plausible headcanon that Osamu and Ryou were intended to own the two halves of Mille’s whole. 

So you might be thinking, Iggy, your Osamu doesn’t have a digimon, how is this reconciled with your main verse? The answer is that it doesn’t really affect it. In the default verse, Osamu dissects the digivice and the Gennai Order decides that’s bad, so he’s blacklisted from digimon work and barred from entry to the digital world. This doesn’t mean that he couldn’t have had an assigned partner before that point. Just that they’d never meet.

The divergent point in the timeline that allows this new, fun, wildly negligent of canon AU to occur is simply that he does in fact go to the digital world as the Gennai Order intended. Obviously there was some kind of danger around this time that they felt a need to try and summon a child in the first place, but whatever it was mustn’t have been very worrisome in the end, considering they made no further efforts. 

We’ve decided that this occurs within the same summer of 1999 wherein Adventure happens, but a bit earlier. Of course, in digital world time, this means years and years, and within the spaces that Osamu is absent, his partner grows to be the Mugendramon also seen in Adventure.

Mugendramon has already been a Dark Master in at least one iteration of self before this, seeing as the original chosen also fought him, so he’s got a pretty good idea of what he wants to do, and he sets up an evil base in the city shown during episode 48-49 of Adventure. Osamu doesn’t have the emotional energy to care about his partner’s crimes, what with his rigorous and spiritually taxing schedule forced onto him by his parents, and he mostly uses his digivice to take extended depression naps in the faster paced time of the digital world. 

Osamu and Mugen aren’t the mushiest of partners, but they have a symbiotic relationship nonetheless. Osamu is an infinite battery for evolving digimon, Mugen provides a safe space and an ear to talk or cry into if necessary.

At some point, he starts bringing Ken along to have more playtime with him, Ken finds a little Minomon floating around the base and gives his Niichan some puppydog eyes as he begs to keep it.

Also they steal Ryou’s partner’s egg and kidnap him to play in the base.

Moving on, Adventure happens, Mugen is reformatted and loses his status, and worse still, time is reset to match the real world’s, so Osamu’s newly adopted coping mechanism is ripped away from him, and his depression hits an all time low as he can no longer rest or play with Kenny or enjoy life to any extent.

Then the accident happens around October.

The Anode/Cathode game doesn’t happen, because Milleniummon doesn’t exist as a villain and nothing in that game can exist in separation from him.

Tag Tamers does, the next summer, the day after Bokura No War Game, because the villains here can exist independently of Mille. There are changes, though. Osamu and Ryou aren’t separated into split worlds like Ken and Ryou are in canon because that was a Mille thing. They also never get to keep any of their recruits from dungeon to dungeon. As there is no overarching threat that’s going to kill everything forever, the digital world’s motive to aide their quest extends as far as the locals not wanting their villages to be ransacked, and said villagers abandon the boys as soon as they’ve served their purpose in that location. Also Osamu has to be carried around like a baby, lol. Ken gets relegated to his position as sentient pokedex at this point, too.

Additionally, since time is never thrown back out of sync with the real world by the events of Anode/Cathode, this game happens over the span of Real Months, so both of the boys are now missing children as well as being very stressed out about the time they’re losing.

Unfortunately for them, the Gennai Order thinks having the timelord jogress around to solve all their problems for them is just a bit too convenient to throw away. So they don’t get to go home like they’d been promised they could.

Ryou gets his usual amnesia/mutism bundle so that he only remembers how to be a good boy who follows orders, but since Osamu isn’t really instinctively predisposed to obedience, and his motives to fight are namely through loyalty to Mugen, helping his partner’s world not die, and to finish the war as quickly as possible so he can go home to his brother, other methods are used.

If memories can be erased, they can be disoriented. Osamu develops a very skewed sense of time passing, so that every time he asks when he can go home, they can just say “in a few more battles” each time and arouse no suspicion or defiance. Ken is blocked from messaging his brother and replaced with a bot that mimics him and convinces Osamu that Ken is still seven and happy and patiently waiting for him, while his real brother is desperately fighting for a way to get his family back.