SAP BO supports a list of database type (Please refer SAP BO doc for the list of support db's), but the default it uses is SQL Server. BDSDA docs specify the same. BDSDA recommended it since SQL Server is used as the default database for SAP BO.
However you can still install SAP BO to use Oracle or any other database. All is that you need to create seperate schema for CMS and Audit and point them during SAP BO Installation.
I had a chance to set this approach in-house and it works.