Line a 4- to 6-quart slow cooker with parchment paper.

Grease the parchment paper with the olive oil.

In a bowl, whisk the eggs with sea salt and black pepper.

Add the feta cheese, tomato, onion, black olives, and oregano. Stir to combine.

Pour the egg mixture into the prepared slow cooker.

Cook the casserole on LOW for 4 hours, until the eggs are cooked through.