TFP x TFO AU where the Orion Pax Arc happens, and rather than getting his memories reset back to shortly before he was named Prime by the counsel (notably not made an actual Prime by receiving the Matrix, which happened later), Optimus's memories get reset back to when he first got his transformation cog, since that would be a relatively equivalent position of 'not a Prime but associated enough to count' cuz the cog came from one of the Primes.
As such, Orion doesn't know about Megatron dropping him off a cliff or killing Sentinel, though he does know what Sentinel did and that D-16 was VERY upset about it/blamed Orion for them finding out, so Megatron is able to do his whole thing about blaming the war on someone other than himself and convince OP to come with him. Maybe he blames Bee or something like 'oh there's actually a really good reason he was sentenced to sub-level 50 because he was actually an evil mastermind and used the happy, talkativeness as a front'.
Anyway, Orion goes with Megs, but the thing is, even though Orion snuck into the archives and messed around with the files, he doesn't have the decoding/archiving skillset that TFP Orion had, so having Orion decode the Iacon database wouldn't make as much sense. But you know what the Decepticons do need that they spend a lot of time doing on Earth that Orion Pax at the point in time of his memories had spent most of his life doing?
So instead of locking Orion in a room and having him guarded there, he tells them that on this new planet, they need energon badly, so for now Orion needs to help in the mines. Conveniently for Megatron, this also means that there will always be other Decepticons present to monitor Orion and make sure he doesn't get into trouble.
Nevermind the fact that the reason Orion got into trouble in the first place was specifically to prove cogless bots (which he no longer is anyway) were capable of more than just mining, and Orion is well aware that something is wrong with his friend. His last memory before waking up with D/Megatron in that cave and fleeing through the groundbridge was D-16 ranting at him just before they got their cogs, and now Megatron's eyes are red. Not to mention, D-16 of all people would know just how much Orion hated being forced to mine, and now here he is, dumping Orion into a mine on an alien planet with no one he knows and no way to contact anyone else.
He wants to trust his friend, but he can tell he's not being told something, and he only gets more suspicious when the Autobots break into the mine (whether to rescue him directly or to get energon and just happen to also find him mining in the process. Soundwave doesn't get the chance to groundbridge Arcee away cuz Orion wasn't cooped up in a room on the Nemesis), and he recognizes Arcee as a fellow former-cogless bot fighting for the Autobots, which he figures wouldn't really make sense. He knows Arcee and she seemed happy to see him and also sad and also not evil? But Megatron shows up and the Autobots have to retreat. Meanwhile Megatron seems really angry and EXTREMELY insistent that Orion just mine, and that that's his role in the Decepticon army.
And that causes something in Orion's conviction to trust his best friend to break. So as soon as he's given the opportunity, Orion does what he does best and sneaks back onto the Nemesis to look for answers, and oh boy does he not like what he finds