Cook the asparagus in boiling salted water for 3 - 4 minutes, until just tender. Drain and refresh under cold water.
Grate the zest from half an orange and reserve. Peel both the oranges and cut into segments. Squeeze out the juice from the membrane and reserve the juice.
Put the asparagus, orange segments, tomatoes and lettuce into a salad bowl. Mix together the oil and vinegar and add 1 tbsp of the reserved orange juice and 1 teaspoon of the zest. Season the dressing with salt and pepper. Just before serving, pour the dressing over the salad and mix gently to coat.