It is possible to provide users access to multiple workgroups without giving the user "view all" access.
Open the user's account under Configuration > Users. The workgroup ID field on the general tab is the default workgroup for the user. New transactions created by the user will default to this workgroup. The default customer and manufacturer list will also correspond to the default workgroup for the user.
Select the Security tab and revoke all "view all" access for the user:
Select the "Workgroup Access" tab for the user. The user will always have access to their default workgroup. Check any additional workgroups the user has access to.
Note: The user will need to log out and back into OASIS for this change to take effect.