Preheat oven to 375 degrees F.

Heat the oil in a heavy pot over high heat. Add the onion and saute until translucent, about 3 minutes.

Add the spinach, in batches if necessary, and saute until just wilted.

Lightly beat the ricotta and eggs together in a bowl.

Add the spinach mixture, feta, oregano, and salt and pepper to taste and mix thoroughly.

Pour the batter into a greased 9x13-inch pan. Bake for about 40 minutes, until the top is very lightly browned. 

Serve warm or at room temperature.