Holy Oil

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.

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 truck of the Impala, each of which was presumably supplied by Castiel.

It is also discovered that Holy Oil is one of the components needed in order to see hellhounds. Dean scorches two pairs of glasses in Holy Oil fire.

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.

Uses of Holy Oil

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 somehow. (5.03 Free to Be You and Me)	Sam and Dean use it to trap the Trickster, forcing him to reveal that he is actually the Archangel Gabriel. Dean releases him by turning on the fire sprinklers. (5.08 Changing Channels)
Lucifer uses it to trap Castiel, but he manages to break the circle and escape by throwing Meg on the flames. The extent of her injuries was unclear, but she survives. (5.10 Abandon All Hope...)	Castiel - with a cry of "Hey Assbutt" - uses a molotov cocktail of Holy Oil to momentarily banish Michael. (5.22 Swan Song)
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. (6.03 The Third Man)	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. (6.20 The Man Who Would Be King)