Tested with VirtueMart versions:
2.6.x - 3.6.10
In the Joomla admin panel, go to Extensions -> Manage in order to install the OPAY VirtueMart plugin.
In the Payment Method field, select VM Payment - OPAY.
Fill in the mandatory fields under Main Settings and Save the payment method again. If your online store is not registered with the OPAY system, fill out the registration form to receive all the necessary configuration data.