When the channa is large enough it will probably eat the oscar, but if to small the oscar might eat it, bad risk either way.
Fishbase (usually pretty reliable) reports them as maxing out at around 130 cm., and 20 kilos.
They also list them as harmless, which only means that they have not confirmed any report of serious injury or death caused by them.
This is opposed to fish such as the red bellied pacu which is listed as traumatogenic for it's propensity for removing fingers and for causing two known human deaths.
That being said, they are widely spread in an area that the news is often under reported, and the fish is very aggresive in defending it's brood. I have seen bowfins strike boat paddles in similar circumstances, and even the little giribaldi will strike at diver to protect a nest. The snakehead could concievably kill a human if it bit them right. The two people killed by pacu were in New Guinea, using hand seines to remove adult pacu from an aquaculture pond, they were naked and the pacu emasculated them so they died of blood loss and shock. A large channa could easily do the same.