Here is the code in my python file
class InheritSaleOrder(models.Model):
_inherit = 'sale.order'

overall_discount = fields.Boolean('Remise globale')
amount_overall_discount = fields.Float(string='Montant de la remise globale', invisible=True)

In my xml file:
<?xml version="1.0" encoding="utf-8"?>
<odoo>
<record id="view_overall_discount" model="ir.ui.view">
<field name="name">view.order.form.inherit</field>
<field name="model">sale.order</field>
<field name="inherit_id" ref="sale.view_order_form"/>
<field name="arch" type="xml">
<xpath expr="//form//sheet//notebook" position="after">
<field name="overall_discount"/>
<field name="amount_overall_discount"
attrs="{'invisible': [('overall_discount', '=', False)]}"/>
</xpath>
</field>
</record>
</odoo>

When I run it, it doesn't show the text "Remise globale" and "Montant de la remise globale" in the view.

To see label (name of field) of your fields you must insert them inside <group></group> tag. Or add label tag seperatly:
<label for="your_field_name"/>

I solve it by putting all my fields in a group. Thanks for your help.

Yes that's what I did, thanks.