Oscars are very easy to keep. 9/10 times hith is caused by bad water quality, poor diet, or both (feeding goldfish & other ridiculous things like that). I'll detail it out below from my experience.
Bare bones minimum for 1 oscar no tank mates = 55gal (75gal much more ideal especially at 12in+)
Plenty of filter gph ( I recommend 10x your tank volume per hour so a 55 would = 550gph)
Plenty of bio media ( scrap carbon) use some pillow stuffing as your 1st stage of media then shove bio media everywhere else where it will fit after that.
Diet wise: I recommend nls or omega one for an oscar (make sure pellet is the right size for your fish).
Feedings: 2-4in feed twice a day just till the belly gets slightly rounded, 5-8in feed once a day till belly gets slightly rounded or agression declines ever so slightly, 9in+ feed 5-6 days per week just till agression declines slightly. Green peas are your best friend it's a natural laxative & will keep things regular.
Waterchanges: Keep nitrates below 20ppm & do a big waterchange every week ( I recommend 75% per week)
Water temp: I recommend 80F, but 78-80 will be fine.
Probiotics: I use a dose of rid-x every month (can't remember exact but just divide max gallonage & adjust to your tank size)
Pothos plants: Ivys hung in the back of your tank help reduce nitrates & look nice imo. Optional, but helps a lot.