Requirements:

• php with zlib support
• files_external app must be enabled ❗

• Get the code

$ cd /server_path/apps
$ git clone https://github.com/hevelius/files_external_onedrive
$ cd files_external_onedrive
$ composer install

• Activate the files_external app from NC settings page or cli

$ cd /server_path/
$ php -f occ app:enable files_external

• Activate files_external_onedrive from NC settings page or cli

$ cd /server_path/
$ php -f occ app:enable files_external_onedrive

• Fill up the storage details (See Below Configuring OAuth2)
• Fire up the files page to see the OneDrive mounted as external storage

Connecting OneDrive is a little more work because you have to create a onedrive app.

• Log into the https://portal.azure.com/ page and search App Registration

• Click New registration
• Name Your App
• Make sure to select:
  Accounts in any organizational directory (Any Azure AD directory - Multitenant) and personal Microsoft accounts (e.g. Skype, Xbox)

• For Redirect URL (optional) add the following URL http://path/to/nextcloud/index.php/settings/user/externalstorages (Replace http://path/to/nextcloud/index.php with you valid Nextcloud installation path, ensure that you don't leave index.php in the path. Example: https://nextcloud.myserver.com/settings/admin/externalstorages)
• Click Register to create your app.

• Under the section Certificates and secrets add new client secret, Name, choose expires:never.
  Make sure to copy the created key (you will not be able to do that later on). Key needs to be pasted in Nextcloud in last step

• Under the section API permissions
• Click add a permissions
• Choose Microsoft Graph, Delegated permissions
• add auth for [ User.Read | Files.ReadWrite.All | offline_access ] (the last is necessary to perform a correct token refresh)

• Copy Client Id and client secret then Go to Nextcloud /settings/user/externalstorages and Add a new storage OneDrive
• Fill the details Client Id, Client Secrets from you onedrive App page
• Click Grant Access and then you will be redirected for OAuth login
• After completing the OAuth you will be redirect back to Storage Section and you should see green mark along your storage configuration

References:

• https://github.com/NastuzziSamy/files_external_gdrive
• https://github.com/icewind1991/files_external_dropbox