Trouble setting up Nagios to monitor Oracle services
As Phil mentioned, I created $ORACLE_HOME/network/admin and tnsnames.ora.  I have something like this:

OFFDB1 =
  (DESCRIPTION =
    (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)(HOST = oracle-server)(PORT = 1551))
    )
    (CONNECT_DATA =
      (SERVICE_NAME = OFFDB1)
    )
  )


I then modified the monitoring commands:

/usr/local/nagios# libexec/check_oracle_health --connect=OFFDB1 --username user 
    --password "pass" --mode tablespace-can-allocate-next --warning 20 --critical 30