The knife will probably be fine if it is not too small and has plenty of driftwood to hide under. They prefer frozen foods and feed mainly at night. Before bed, feed your oscar, turn off the light, and then offer the frozen food to the knife to make sure that he gets food. Good luck! It is always a gamble:)

As long as your knife isn’t small enough to be a morsel for the oscar and there are enough hiding places for it to be comfortable in the tank, I see no problem with it.

I dont see a real problem with keeping the 2 fish in the same tank.
your tank is big enuff to keep the 2 fish.
the only thing is oscars are funny as it starts to get big it may pick on any other fish and then again it may never care that the other fish is there. some oscars fight everything and some just dont care.

it shouldnt be a problem of your tank is big enough. oscars are quite aggressive

it should be fine, but add hiding places like pumbling pipes or ghost pipes.

My largest black ghost was not 1 foot, it was nearly 2 feet. around 21 inches long.

The oscar will reach about 13-15 inches long.

They are usually ok together, as like you said they are in different zones, but i would include a hiding places for the ghost.

they will be fine together but just to be careful put a lot of hiding places in the tank but for a hundred gallon they should be perfectly fine i say go for it