Slice off the ends of each orange and place the oranges on a cutting board. Using a sharp knife and working from top to bottom, cut off the peel and pith in strips. Working over a salad bowl, hold the orange and make a cut on either side of each segment to release from the membrane, letting the segments fall into the bowl; place the cut oranges in a medium bowl.
Squeeze the remaining juice from the oranges into the bowl; whisk in the vinegar, mustard and honey, then the olive oil and poppy seeds. Season with salt and pepper.
Add the spinach to the orange segments and toss with the dressing.