To understand the permissions required for the MSSQLServerOLAPService and SQL Server Agent service accounts, you need to understand the security context in which various operations are executed. Certain tasks are performed in the context of the logged-on user, and other tasks are performed in the security context of the MSSQLServerOLAPService service account. When you request that Analysis Manager create a new object or browse for an existing object, the task is executed in the security context of the interactive user account of the user performing the task. When Analysis Services processes dimensions, partitions, and mining models, this task is performed in the security context of the MSSQLServerOLAPService service account. You must grant sufficient rights to this service account to ensure that processing will succeed. It is not uncommon for users to assume that if they can create objects, then they can process them. You must ensure that the MSSQLServerOLAPService and the SQL Server Agent service accounts have sufficient permissions for the tasks they need to perform. At a minimum, the service account must be a member of the OLAP Administrators group. This permission is needed for any user (or service that is acting on behalf of a user) that manages an Analysis Services server.
Analysis Services SSAS, dbDigger, Security and Permissions