From the OneDrive for Business Group Policy perspective, two controls are available:

• Allow syncing OneDrive accounts for only specific organizations: By using this setting, you only specify the tenants that your managed OneDrive clients can synchronize with. 
• Block syncing OneDrive accounts for only specific organizations: By configuring this setting, you are only specifying the tenants that your managed OneDrive clients cannot synchronize with.

These settings are not designed to work together. If you specify both, the settings in Allow take precedence. To use either of these settings, you need to enter the tenant ID for the organization(s) you wish to either allow or block. You can locate your tenant ID in the Azure AD Admin Center (https://aad.portal.azure.com/#blade/Microsoft_AAD_IAM/ActiveDirectoryMenuBlade/Properties) under Directory ID. Once you have the tenant ID, you can configure either of the group policy options as follows:

1. Locate the Allow syncing OneDrive accounts for only specific organizations or Block syncing OneDrive accounts for only specific organizations Group Policy object under Computer ConfigurationPoliciesAdministrative TemplatesOneDrive, as shown in the following screenshot: 

2. Edit the setting.
3. Select the Enabled radio button.
4. Under Specify tenant ID, select Show...:

5. Enter one or more tenant IDs (one per line). When you're done, click OK:

6. Click OK to save the entry.

Once configured, OneDrive clients will be limited to syncing with the organizations that have been specified.