~2 Billion row fact table in star schema on Oracle. Advisable?
Row count isn't a great indicator of database size. I would not be worried about Oracle scaling to 2 billion rows. Relational databases can practically scale up into the many terabyte range. The ability to scale comes down to data model, hardware, and developer skill, user requirements, and budget. It's feasible to scale an Oracle (or SQL Server) data warehouse up into the 10s to 100s of terabytes.

Using a star schema is the right starting point for a relational data warehouse. There are a number of well thought out techniques for data warehousing outlined in The Data Warehouse Tookit. This will help you design a data warehouse that can scale.

At some point, you'll want to look into features that your vendor offers like partitioning, materialized views, and dimensional hierarchies. You should be able to put that off in your first attempt, though.