Place orange zest, orange juice, balsamic vinegar, honey, mustard, ginger, garlic, salt, black pepper and red pepper flakes (if using) in a blender container and process until thoroughly combined. With the machine running on low speed, slowly add olive oil through the top tube.
Alternatively, place all ingredients, except the oil, in a glass container and whisk together with a fork. Add olive oil, seal shut and shake the container vigorously.
Serve immediately or refrigerate in a sealed container up to 2 weeks.