Tbh I do not know if Oberon was the one who killed Banana, but as I'm seeing the game right now,
Banana was sheeping and therefore would've been a threat for everyone.
I'm just assuming it's not very likely that the killer is among the ones that Banana sheeped (gyrlander or frinckles idr).
All of this is still some speculation, but that's all we got for now regarding Banana's death, I believe.