| I am sorry to hear that. What I write below is not intended to be hostile, just to help you out a bit.
If your tank info is correct, your problem is not the Oscar but likely one or all of the following:
a) the severe overstocking you have there. A 55gal is barely adequate to keep one Oscar and zero tankmates. Your tank info states that you are keeping 4 Heros Severums (one Gold, three Green), three Convicts, one Angelfish, one peacock, three bala sharks, one pleco, one green terror and no idea what tiwan or polipaturus african mix means. This is just way too much for a 55gal tank.
b) the combination of three Oscars in one tank is mostly asking for trouble unless you keep three females and two of them don't pair-off (they do sometimes), which is quite hard to tell. Same goes for three Convicts except that female Convicts won't pair-off.
c) to say the least, questionably safety in tankmates selection.
I suggest you read around the profile of each of your fish, check their minimal tank suggestions, water parameters... and please take a minute to reconsider what you are trying to do by keeping all this fish in a 55gal tank.
I have made similar mistakes as this myself (e.g. 29g bow = footprint of 20gal , in June 2008 I kept temporarily there 6 Convicts juveniles with two sub-adult Oscars and three juvenile Kenyi as I was setting their big tank. Two Convicts were viciously killed. The four Convicts, two Oscars and three Kenyi went into the big tank and no problems whatsoever due to the huge space and hiding they had)
Pepe
Santo Domingo | 
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