“Oasis Workflow” plugin uses the standard wp_mail() function which in turn uses mail() function to send out emails.
If you are having issues with emails not being sent, we suggest to setup a SMTP Email plugin. These plugins reconfigure the wp_mail() function to use SMTP instead of mail(). There are quite a few SMTP plugins – like https://wordpress.org/plugins/wp-mail-smtp/, https://wordpress.org/plugins/easy-wp-smtp/ etc.
This has solved the notification mail issue in almost all the cases.