WebLogic can connect to oracle RAC either through weblogic multi pools or through Connect-Time failover (underlying JDBC driver will be responsible for failover)
In case of Multi Pool/Multi Datasource, each pool/datasource is assigned to one RAC node and weblogic JDBC framework would make sure of loadbalancing/failover. For WLS8.1, this can be done by configuring multi pools and from WLS9.x through Multi datasource.
In case of Connect-Time failover, underlying JDBC driver will perform the failover under failure conditions.
Transaction handling would different based on whether the transaction is local or Global( XA transaction with Prepare/commit phases).
As for global transaction, weblogic does not provide transparent failover to database connections.
If RAC node fails before COMMIT phase, application would receive an exception and this has to be handled in the application by getting a new connection and attempting to process the transaction.
There is good article that provides step by step instructions on setting on weblogic multi datasource with oracle RAC