This is depending you Sale order Invoicing methode.

Se the order_policy:

        'order_policy': fields.selection([
            ('prepaid', 'Pay before delivery'),
            ('manual', 'Deliver & invoice on demand'),
            ('picking', 'Invoice based on deliveries'),
            ('postpaid', 'Invoice on order after delivery'),

If you choose the 'Deliver & invoice on demand' you can invocing directly.

and so on: you can find a lot of manual about this:

https://doc.openerp.com/6.0/book/3/3_7/invoicing/