Oracle RAC can you have a cluster that contains one node
From the documentation:


  Oracle RAC One Node is a new option available with Oracle Database 11g
  Release 2. Oracle RAC One Node is a single instance of an Oracle RAC
  enabled database running on one node in a cluster. This option adds to
  the flexibility that Oracle offers for database consolidation while
  reducing management overhead by providing a standard deployment for
  Oracle Databases in the enterprise.


This is a means to have a multi-node RAC, one which one database can only run on one node at a time, so it allows you to (more cheaply) use RAC for active-passive clustering, which saves on having both RAC and another clustering product such as VCS. Or are you asking if it is possible to have a RAC system entirely on one physical host, running Clusterware, ASM, etc and a database? Yep, that is possible too, the standard installer can do this for you.