Finely shred 1 1/2 teaspoons peel from orange. Peel, seed, and section orange, reserving juice. In a food processor or blender combine the shredded orange peel, sections, and juice; the vinegar; sugar; mustard; onion; and pepper. Cover and process or blend until well mixed.
With the processor or blender running, slowly add oil in a steady stream until mixture is thickened. Transfer to a small bowl. Stir in poppy seeds. Store in the refrigerator for up to 1 week. Stir well before using.