Holy oil is a rare substance. Castiel obtains some from Jerusalem, in order to use it to restrain Raphael. When the oil is ignited, its flames will trap angels and archangels, if an angel touches or attempts to pass through the flames they will die.
Sam describes it as "a devil's trap for angels" but it differs because angels can still use some of their powers while trapped within the flames. Sam and Dean keep bottles of the oil in the trunk of the Impala, each of which was presumably supplied by Castiel.
Holy fire is capable of hurting, though not killing demons. Castiel throws Meg onto a ring of holy fire to escape and Sam douses Abaddon in holy oil and sets her on fire. Both are not killed, but Abaddon is left in so much pain she abandons her meatsuit and flees. However, a dissatisfied demon is able to use it to commit suicide to make a point. 
- 1 Episodes
- 1.1 5.03 Free to Be You and Me
- 1.2 5.08 Changing Channels
- 1.3 5.10 Abandon All Hope...
- 1.4 5.22 Swan Song
- 1.5 6.03 The Third Man
- 1.6 6.20 The Man Who Would Be King
- 1.7 8.14 Trial and Error
- 1.8 8.23 Sacrifice
- 1.9 9.01 I Think I'm Gonna Like It Here
- 1.10 9.18 Meta Fiction
- 1.11 9.18 Meta Fiction
- 1.12 9.22 Stairway to Heaven
- 1.13 10.03 Soul Survivor
- 1.14 10.18 Book of the Damned
- 1.15 11.01 Out of the Darkness, Into the Fire
- 1.16 11.02 Form and Void
- 2 References
Castiel and Dean trap the archangel Raphael inside a ring of burning holy oil and ask him for God's location. He claims that God is dead, and refuses to cooperate. They leave him in the still burning circle. He eventually escapes.
Dean traps the angel Balthazar in a circle of holy oil and forces him to return the soul he bought from a boy, Aaron Birch, with a piece of the Staff of Moses. Castiel causes the circle to vanish with a gesture when he wishes to free the angel.
Dean, Sam and Bobby trap Castiel in a circle of ignited holy oil and force him to admit to working with Crowley. Castiel tells Dean, Sam and Bobby to leave him when the sound of an approaching storm of Demons is heard. They leave, and only Crowley enters; he causes the circle to vanish with a gesture, freeing the angel.
Searching the demon tablet for a way to see hellhounds, Kevin Tran finds out that an object scorched by holy fire can be used to view them. Dean ends up scorching two pairs of glasses with holy fire which allows him and Sam to see Crowley's hellhound and kill it.
Sam and Castiel find the angel Josiah badly burned by holy fire. After falling for the trap laid out by Metatron, Josiah was doused with holy oil and set aflame by a flame thrower hanging over the entrance to what he thought was a doorway to Heaven.
In protest to the way that Crowley had been running Hell, a demon douses himself in holy oil and immolates himself in front of Crowley.
When preparing to destroy the Book of the Damned, Dean adds holy oil to the fire. However, Sam only pretends to burn the book.
Dean is shown throwing a container of holy oil into his duffel bag when he is getting weapons out of the Impala's trunk.
After Billie tells Sam that he is "unclean in a biblical sense" due to his rabid infection, Sam comes up with the idea of using a biblical purification as a cure and searches the Bible online for such a purification. He finds indicators that holy fire can be used for such a purpose and douses a sponge in holy fire before using it on himself. The holy fire burns away the rabid infection and cures Sam. Now with a viable cure for the rabids, Sam lures a group of three into the hospital chapel where he traps them in a ring of holy fire. Proximity to the holy fire causes the rabid infection to burn away and the people to regain their humanity. Sam then recruits the people to use the holy fire cure on the remaining people of the town who survived becoming rabids.