The following code prints the label "Invoice Date" and the actual date in separate lines.
So,
<div class="col-xs-8" t-if="o.date_invoice">
<strong>Invoice Date:</strong>
<p t-field="o.date_invoice"/>
</div>
Generates:
Invoice Date:
05/05/2015
What should be done to generate:
Invoice Date: 05/05/2015
(just one line)