Hi. you need to read this...
tank cycling
Now, the oscar that died..did he die before you got the new one?
Do you have a water test kit? If not, get one..the liquid kind.
How big is the tank?
Are there Tank mates?
What sort of filter set up do you have?
What is the temperature?
How long did you have the other oscar?
Was he sick before he died?
You need to post the water data after you test it..
Ammonia, Nitrites, Nitrates, pH
Do this ASAP so that the experts on this site can tell you if your new oscar is in danger.
Get as much info posted as you can.
Good Luck
Sharyn