General Antonyms:
* Truth
* Fact
* Reality
* Authenticity
* Veracity
* Legitimacy
More Specific Antonyms (depending on context):
* Genuine (if the hoax is about a product or item)
* Honest (if the hoax is about someone's actions or statements)
* Credible (if the hoax is about information or a story)
* Real (if the hoax is about something tangible)
* Truthful (if the hoax is about a statement or claim)
The best antonym will depend on the specific context in which "hoax" is used.