During installation, OUI detects if you have Oracle Grid Infrastructure for a cluster installed. If you do, then you must specify the type of database you plan to create:
A single-instance database
An Oracle RAC database
An Oracle RAC One Node database
If you plan to create databases of different types on this cluster, then choose the most advanced option. For example, if you plan to create only single-instance and Oracle RAC One Node databases, then choose the Oracle RAC One Node database option. If you plan to create single-instance databases and Oracle RAC databases, then choose the Oracle RAC database option.
You can install Oracle RAC One Node on two or more nodes in the cluster. An Oracle RAC One Node installation starts up an instance on one of the nodes you select as an Oracle RAC One Node pool member. If that instance goes down, then the Oracle RAC One Node instance fails over to another pool member. This feature relocates database instances and connections to other cluster nodes for high availability.
Oracle Database High Availability Overview for information about Oracle RAC One Node
Oracle Real Application Clusters Administration and Deployment Guide for information about how to convert single-instance databases to Oracle RAC