Can oscars live with black ghost knives?

Verdict
Yes

Yes. Oscars and black ghost knives get along in the same tank. Both are fine at 74–81°F. Plan on at least 125 gal.

OscarOscar74–81°F · pH 6–8 Black ghost knifeBlack ghost knife73–82°F · pH 6–8

Why it works

Water side by side

55°60°65°70°75°80°85°90°Both: 74–81°Oscar 74–81°Black ghost knife 73–82°
The green band is the temperature range both can live in.
456789Both: 6–8Oscar 6–8Black ghost knife 6–8
The green band is the pH both tolerate.

Size to scale

Oscar · 12 inBlack ghost knife · 15 in0"2"4"6"8"10"12"14"
Adult length in an aquarium. A fish can swallow anything that fits in its mouth, roughly a third to half its own length for predators.

Care at a glance

OscarBlack ghost knife
Temperature74–81 °F73–82 °F
pH6–86–8
Adult size12.0 in15.0 in
Minimum tank75 gal125 gal
TemperamentSemi-aggressiveSemi-aggressive
Swims atMiddleBottom
GroupAlone or in pairsOne per tank
DietCarnivoreCarnivore

Better tank mates for oscars

All oscar tank mates

Better tank mates for black ghost knives

All black ghost knife tank mates

Questions

Can oscars live with black ghost knives?

Yes. Oscars and black ghost knives get along in the same tank. Both are fine at 74–81°F. Plan on at least 125 gal.

What temperature suits both oscars and black ghost knives?

74–81 °F. Oscars need 74–81 °F, black ghost knives need 73–82 °F.

What size tank do oscars and black ghost knives need together?

At least 125 gal, set by the black ghost knife. Add space for each extra fish.