Based on the Grid Infrastructure, RAC One Node is a RAC Database with only one instance running. This architecture reduces unplanned as well as planned downtime. If the server crashes that holds the instance of a RAC One Node database, that instance can failover to another node in the cluster automatically. If we need to take down the server (or the clusterware stack there) because of a maintenance task, we can do an Online Database Relocation:

The demo was done with 11gR2 and it is very similar to what I show live during the Oracle University course Oracle Database 11g: RAC Administration about RAC One Node. Hope you like it 🙂