from odoo import models, fields

class Order(models.Model):
_name = 'order.model'
_description = 'Order'

name = fields.Char(string='Order Name', required=True)
order_lines = fields.One2many('order.line.model', 'order_id', string='Order Lines')

class OrderLine(models.Model):
_name = 'order.line.model'
_description = 'Order Line'

order_id = fields.Many2one('order.model', string='Order')
product_id = fields.Many2one('product.model', string='Product')
quantity = fields.Float(string='Quantity', default=1.0)

and view
<odoo>
<data>
<record model="ir.ui.view" id="view_order_form">
<field name="name">order.model.form</field>
<field name="model">order.model</field>
<field name="arch" type="xml">
<form>
<sheet>
<group>
<field name="name"/>
</group>
<notebook>
<page string="Order Lines">
<field name="order_lines" mode="tree">
<tree editable="bottom">
<field name="product_id"/>
<field name="quantity"/>
</tree>
</field>
</page>
</notebook>
</sheet>
</form>
</field>
</record>
</data>
</odoo>