Cut off the top and bottom of the oranges to expose the fruit and place cut-side down on a cutting board. Using a knife, cut off the peel and white pith, following the curve of the fruit. Holding the oranges over a bowl, cut in between the membranes to release the segments into the bowl.
In a large salad bowl, whisk together the lemon juice and olive oil. Add the greens and toss lightly to coat them with the dressing. Season with 1 teaspoon each salt and pepper. Add the orange segments. Serve immediately.