Tarot has a structure that can be applied, to greater and lesser degrees, to most tarot decks. There will be variances, but that is a general rule of thumb. Tarot decks generally have 78 cards divided into the Major Arcana, then the Minor Arcana, further divided into the four suits. This is not set in stone, but is a generalization.
Most oracle decks have a structure, or no structure, that is unique to them, unless they are a specific type of oracle, such as lenormand or Sybilla (or playing cards).
I prefer tarot to oracle in general because I like the structure of tarot, and I like to not have to look at the guide book, although there are plenty of oracles meant to be read intuitively, and plenty of people who read them that way, meant to or not.
That being said, all tarot decks ARE oracles, like all zebras are animals, but not all animals are zebras. Perfectly clear, right?