Opal Guardian — ruling 1

A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Opal Guardian — ruling 2

Guardian's effect doesn't wear off.

Opal Guardian — ruling 3

After the triggered ability resolves, Opal Guardian is no longer an enchantment.

Opal Guardian — ruling 4

A face-down creature spell will cause Opal Guardian's ability to trigger (if it's an enchantment).

Opal Guardian : règles, fonctionnement et interactions (MTG)

Opal Guardian is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

When an opponent casts a creature spell, if this permanent is an enchantment, this enchantment becomes a 3/4 Gargoyle creature with flying and protection from red.

How it works : Opal Guardian

Original English text — Wizards of the Coast / Scryfall

#1August 1, 2008· Wizards of the Coast
A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.
No other card is named in this ruling.
#2September 25, 2006· Wizards of the Coast
Guardian's effect doesn't wear off.
No other card is named in this ruling.
#3September 25, 2006· Wizards of the Coast
After the triggered ability resolves, Opal Guardian is no longer an enchantment.
No other card is named in this ruling.
#4September 25, 2006· Wizards of the Coast
A face-down creature spell will cause Opal Guardian's ability to trigger (if it's an enchantment).
No other card is named in this ruling.