With a small, sharp knife, cut ends off orange, deep enough to reveal flesh. Set orange on one cut end on a board. Following its curve with the knife, slice off peel and white pith, down to orange flesh. Holding orange over a strainer set over a small bowl to catch juice, cut between inner membranes and fruit to release segments; drop segments into strainer. Squeeze juice from membranes into bowl; discard membranes.
In a large bowl, whisk together olive oil, lemon juice, and 1 tablespoon of the orange juice; reserve remaining juice for another use. Add watercress, orange segments, and almonds. Mix gently. Add salt and pepper to taste and mix again. Mound on plates.