If you go past the Silverite mine and carry on, don't pass the blood lotus, it's on the rocks there.
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The oil is in the kitchen, in the inn in the city of Amaranthine.
In the Silerite Mines in with the dragon tamer, but be careful you can only get there once so remember to grab it
Nowhere. The only DLC that features Morrigan is "Witch Hunt".
From the Dragon Age Origins main menu, there's an option called 'Other Campaigns' which is where you launch the Golems of Amgarrak and Witch Hunt adventures. When you select one, you'll be given the option to create a new character or import from another module. You'll be able to import your character from any previous save, including a character from Origins who was imported into Awakening.
Loghain makes a cameo appearance if he was left alive during Origins. He will give you a gift of a decent axe, though it's one you can buy in Amaranthine.