Hi Alfonso, welcome to oscarfishlover!
What type of fish did your 2 to 3 inches Oscar ate last week?
Was it a feeder's fish (raised for that purpose) or just another fish that was sharing the tank with? What other fish do you keep with your Oscar? What is your tank's volume? Do you know what your water parameters -pH, temp, ammonia, nitrites, nitrates- are?
Many things could explain non-eating behavior in a young Oscar: I read about one Oscar that the boss of this site (OFL) kept, that ate a type of catfish and almost died. A hard spine of that catfish got stuck in the Oscar's mouth. Your Oscar could be stressed, or bloated.
Can you take a picture and post it?
Hope moderators move this tread to the Emergency sub-forum.
Pepe
Santo Domingo