Preheat the oven to 180°C. Grease and line a 23 cm round cake tin.
Place the sugar and orange zest in a large bowl. Rub the orange zest into the sugar until it is damp and fragrant. Whisk in the eggs until combined. Add the orange juice and pour in the olive oil. Add the ricotta and whisk it all together, then gently mix in the flour until just combined.
Pour into the prepared tin and bake for 45 minutes or until a skewer inserted into the middle of the cake comes out clean. Allow the cake to cool in the tin for 10 minutes, then remove from the tin and continue to cool on a wire rack. Once cool, dust with icing sugar if desired, then serve.
The cake will keep in an airtight container for 3–4 days