Enchantment, 4W (5) When an opponent plays a creature spell, if Opal Archangel is an enchantment, Opal Archangel becomes a 5/5 Angel creature with flying and vigilance.1
Illus. Jeff Miracola
When it turns into a creature, it no longer counts as an enchantment. [Urza's Saga Rule Page]
It changes even if the spell is countered. [D'Angelo 1999/05/01]
It becomes a creature when the spell is announced, which is before that spell resolves. A Cloudchaser Eagle cannot be used to destroy this card, since it will no longer be an enchantment when the Eagle comes into play. [Urza's FAQ 1998/10/05]
A creature spell is any "Creature" or "Artifact Creature" spell. [D'Angelo 1999/05/01] Older cards of type Summon are also Creature spells.