class Order(models.Model):
    title = models.CharField(max_length=255)
    body = models.TextField()
    creator = models.ForeignKey('User', null=True, blank=True)

class OrderAdmin(admin.ModelAdmin):
    fieldsets = [....]

    def save_model(self, request, obj, form, change):
        if getattr(obj, 'creator', None) is None:
            obj.creator = request.user
        obj.save()