Tested with WooCommerce versions:
2.0.20 - 5.8.x
In the administration environment, go to Plugins -> Add New and click on Upload Plugin.
Select the OPAY plugin zip archive and click on Install Now. Once the plugin is installed, click on Activate to enable the plugin.
Extract the contents of the OPAY plugin zip archive. Using an FTP client program, upload the extracted files to the WordPress plugins directory (wp-content/plugins).
Go to WooCommerce -> Settings -> Checkout -> OPAY Payment Gateway.
Fill in the mandatory settings of the OPAY plugin, check the Enable/Disable checkbox, and click on Save changes. If your online store is not registered with the OPAY system, fill out the registration form and receive all the necessary data for the settings.