Override default_get method of your wizard and prepare the o2m lines data to show the lines when you open a wizard.

Ex:

class WizMain(models.Model):
    _name = 'wiz.main'

    lines = fields.One2many('wiz.line', 'wiz_id')

    def default_get(self, fields_list):
        result = super(WizClassName, self).default_get(fields_list)
        order = self.env['sale.order'].browse(self._context.get('active_id'))
        # Prepare o2m lines data
        lines = []
        for line in order.order_line:
            lines.append((0, 0, {'product_id': line.product_id.id, 'qty': line.product_uom_qty}))
        result.update({'lines': lines})
        return result


class WizLine(models.Model):
    _name = 'wiz.line'

    wiz_id = fields.Many2one('wiz.main')
    product_id = fields.Many2one('product.product')
    qty = fields.Float()