It's actually more complicated than the Wiki article suggests.
Pufferfish do not synthesise poisons. They absorb poisons from their food, specifically algae inside the guts of clams and other shellfish. When they stop feeding in the wild, the amount of poison slowly declines. There's contradictory lab work on how fast this happens, so probably varies from species to species. But it does happen.
There's also variation in where that poison is located. So far as I know, the poisons are inside the body, including the skin, rather than the mucous on the outside (compare, for example, with boxfish). For your oscars to have swallowed some poison, they must have done rather than more than merely "taste" the fish. Puffers are usually ignored by predatory fish of similar size; these puffers must have been very small, surely, to have been sucked/bitten by your oscars?
I'm assuming these were green spotted puffers, such as
Tetraodon nigroviridis or
Tetraodon fluviatilis. These need brackish water in captivity, so had no business being out in a tank with oscars anyway. I can't think of any pufferfish that would be a safe companion for an oscar, so as ever, we come back to the mantra of research the needs of a fish before purchase.
Cheers, Neale
Ok, it`s a silly question now.....but are they poisonous. I need categorical proof that they are. I just bought 2 from my LFS and my albino and tiger Oscars were very, very inquisitive (they tried to `taste` them).....result, both Oscars now lying on a piece of newspaper...dead!!!! The shop said they`ve never heard of this happening as they`ve sold 100`s but will do some research and find out. I`m not happy and very gutted cos I`ve killed my fish.