Well, that's CK's for you. You should be worried when you turn the lights off. To many Ck attacks happen at night, when there hunting for food and they just strike at anything that they think they can eat. I would recommend removing the Oscar and let him Get bigger in a separate tank, they grow fast. And once he is bigger the Oscar should be Ok.
And if thats not possible, keep everyone Well feed and the Oscars will grow faster than the CK.
A few things can explain this could be from the individual personalities of CK, to the Size of the Tank and feeding habits. And stock, how many fish are running around. Less fish sometime means that more aggressive fish have less targets to mess so they do more damage.