« ORACLE_BASE not set in environment. It is recommended that ORACLE_BASE be set in the environment »
To fix the problem, one simply has to issue a simple command.
Note that this database's unique name is ntsdev.
srvctl setenv database -d ntsdev -t "ORACLE_BASE=/u01/app/oracle"
Once that was executed, I bounced this particular instance using:
srvctl stop instance -d ntsdev -i ntsdev2
srvctl start instance -d ntsdev -i ntsdev2
While restarting the instance, I kept an eye on the alert log using another shell:
adrci> set home diag/rdbms/ntsdev/ntsdev2
adrci> show alert -tail -f
Or if you're not into the new ADR thing, then just use the good old tail command.
tail -F $ORACLE_BASE/diag/rdbms/ntsdev/ntsdev2/trace/alert_ntsdev2.log
Sure enough, once this instance restarted, it told me that the ORACLE_BASE variable was found in the environment:
ORACLE_BASE from environment = /u01/app/oracle
Note that the following is deprecated and is therfore ignored. Oracle is nice enough to tell it to you.
srvctl setenv instance -d ntsdev -i ntsdev2 -t "ORACLE_BASE=/u01/app/oracle"
srvctl setenv instance command has been deprecated and will be ignored.
Should you want to read more about srvctl, then open the « Oracle Real Application Clusters Administration and Deployment Guide 11g Release 2 (11.2) » at Chapter A « Server Control Utility Reference »  and the description of the setenv subcommand in Chapter 3 « Administering Database Instances and Cluster Databases » .