OHKO moves automatically fail if the target has a higher level than the user.
Source. No, the OHKO move will still fail. Accuracy wouldn’t matter here since the game doesn’t consider it if you’re under leveled. Attacking a higher level causes move failure, not reduction in accuracy. You will automatically fail. There’s also this, which says about the same thing but I can’t tell for sure since I can’t access Reddit.
Reddit. Hope this helped!