Hello Mouna Cherrad, for above given scenario,  you can use a compute field and calculate the values from the rows, It will be similar like purchase order's Total(amount_total) field.
for example:

quantity_total = fields.Integer(string='Quantity', compute='_quantity_total')

@api.depends('order_line.quantity')
def _quantity_total(self):
    for order in self:
        order.quantity_total = sum(order.mapped('order_line.quantity'))

Please give more details about what do you want to achieve. You mean before save the record, You can get them in create method from vals. You can override create method and you can get the order_line (one2many) from vals and do what ever you want and then call super method of create/write.