Effective Against
AngelHoly oil is a sacred substance sourced from Jerusalem, highly effective as a weapon against angels. When ignited, it creates a barrier of fire that no angel can cross. A ring of burning holy oil traps an angel as effectively as a devil's trap holds a demon. Direct contact with burning holy oil causes severe injury to angels.
Usage
The most common tactical use is creating a circle of holy oil on the ground, luring or trapping an angel inside it, and then lighting the oil. The trapped angel cannot fly, teleport, or walk through the flame barrier. Holy oil can also be thrown as Molotov cocktails for a direct attack, or applied to weapons to give them anti-angel properties.
Castiel was trapped in holy fire on multiple occasions throughout the series. The substance was also used defensively against hostile angels and during the angelic civil war storylines.
Limitations
Holy oil only affects angels. It has no effect on demons, monsters, or other creatures. The supply is limited — it must come from Jerusalem, and acquiring it requires contacts in specific religious communities. It is consumed when burned, so a single use exhausts the supply. Archangels are more resistant to holy fire than standard angels, though it still causes them pain and can temporarily restrain them.