The order object contains information held against an opportunity record. The order object can be accessed in opportunity documents: 

{{ order.name }}

And within project documents:

{% for order in project.opportunities %}
  {{ order.name }}
{% endfor %}

And within invoices, where an invoice source is an opportunity:

{% for source in invoice.sources %}
  {{ source.name }}
{% endfor %}

From opportunity discussion templates:

{{ opportunity.name }}

Objects

View a list of objects on Google Sheets

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