If you create your invoices via Onfact's cloud invoicing, you can easily receive all these invoices in OkiOki as well.
Go to your settings of your Onfact account and click on the tab 'E-mail settings'.
At the bottom of the page, you can easily add CC: and BCC: addresses for your outgoing invoices. Copy your personal OkiOki email address (found under 'Preferences > OkiOki email address') into the 'BCC Address' field. From now on, any mail that goes out in Onfact will also arrive in OkiOki and pick up the attached invoice. In addition, OkiOki stores all your sales invoices and forwards them to your accountant.
Simple, isn't it?