Usually, adapters are connected by ID/Password login. But you can switch to OAuth2.0 connection by checking Global Resource's [Connect with OAuth] on.
OAuth connection works on authentication flow called "Refresh Token Flow" provided by Salesforce.
OAuth 2.0 Refresh Token Flow
OAuth connection is enabled by using an Access Token which is issued when access from application is permitted by the user. This Access Token is recognized as the Session ID.
However, the Access Token will eventually expire upon the validation date.
An Access Token known as a "Refresh Token" is issued periodically in order to continue to utilize the adapters.
Refresh Token is a type of Access Token which is issued without any actions performed by the applications and users.
It grants permanent access permission to applications until the users turns the setting off from: [Setup]> [Personal Information]> [OAuth Connected Apps]
How to retrieve a Refresh Token
Check Global Resource's [Connect with OAuth] and select its destination from [Connect to OAuth].
Access the URL which you will see on [How to retrieve a Refresh Token?].
Salesforce login page will appear. Login with the permitted ID.
Check the user and its authorization before allowing connection to the adapter.
A new page with the following message will display after granting access permission.
On this page, you will find the Refresh Token embedded as a part of the URL fragment.
Copy directly from the browser's address bar.
What is granted to permitted adapters
Authorizations granted to OAuth access are as below:
Access and manage your data (api)
Perform requests on your behalf at any time (refresh_token)