Section oranges by cutting away the rind and white pith; carefully cut alongside each membrane to remove fruit, using a paring knife. Squeeze remaining membranes to extract juice. Combine juice, vinegar, and next 5 ingredients in a large bowl; slowly drizzle in oil while whisking constantly. Cover and chill up to 2 days (return to room temperature before serving).
Toss lettuce with orange segments, pine nuts, and desired amount of dressing. Serve immediately.