No.
IME piranhas are best alone. No matter what the size, they will always find a way to injure or kill the tankmate. My solo one beat the crap out of a leporinus significantly larger than it and never got a scratch. A school would be unfair.
Anything that walks, swims or flies that come across their path is a potential food item.
If you want an Oscar and a school of piranha look alikes there are several mytinus varieties that are incredible body doubles.
imo oscars are extremely easily bullied. when i was really young i put one in an african cichlid tank. the oscar was 11” the africans 3-4” and the oscar was killed. you need more piranhas anyway so i dont know why you’re considering adding another fish thatll take a substantial amount of space up. ive watched videos where full grown oscars are fed to pirahnas. if you wanted tankmates you shouldnt have gotten pirahnas. the fact that youre already wanting to add other fish is probably a sign that pirahnas arent well suited for you and that youll get tired of them. maybe consider returning them and exchanging them for silver dollars so that you can have a south american themed tank with oscars etc.
pirahnas require as much space as possible as many of their own kind as possible and as much cover as possible. if you have 3 pirahnas and are considering adding an oscar it seems that you are not trying to meet their needs very well at all.
if you have so much experience with piranhas why are you even asking if you could add an oscar into a piranha tank you should know what would happen to it. even if they didn’t immediately rip it to shreds theyd take large chunks overtime.
i know piranhas do fine in small groups but you tend to get more natural or better behavior the more you add and if theres space for an oscar then there must be allot more space for more piranha.
to be considering a large slow moving fish as a tankmate for piranhas then you must have an absolutely gigantic tank but if you have a gigantic tank why would you only have a shoal of 3.