The plugin needs to store the API credentials. You can encrypt them by adding a key in wp-config.php.
First, create a large, random key. Edit wp-config.php and add
define('ONOFFICE_CREDENTIALS_ENC_KEY', 'YOUR CREATED PASSWORD');. Then navigate to onOffice > Settings and save everything to encrypt your credentials.
Secure passwords should be at least 8 characters long, consist of upper and lower case letters and special characters, and should not be found in any dictionary or associated with you.
Note that if you change this key, you will need to re-enter your API credentials.